Several newbie questions

Thank you very much for your help! Most my questions were really newbie ones cause after I've finished first campaign chapter I've got answer to them. But I still have questions even after many hours of play, so I'll very grateful for any help.
1. How I can upgrade general's title?
2. Is anythere a full list of all titles and their abilities?
3. When I upgrade "Personal" item (like Attila's bow) do my general retain all its abilities or only Attila himself?
4. What are mercenaries and what their purpose? Only as general's personal troops? I didn't see any of them from city roster.
5. Can I replace once chosen general's troops? E.g. my Aetius has Brigade troops which are upgraded, so could he muster new troops?
6. Some of general's ranks say "unlock 4 skills", but if general has only 3 skills available will 4th appear or I can add it later from weapon or somehow?

Glad I could kind of help, but yes as you mentioned a lot of the features I feel like are kind of "discovered" if you will, as you play.

As for your further questions:
First I'll answer Charles and then Snowman

1. The title upgrade is available in the same place as the rank upgrade. For some reason that I'm not entirely aware of, only some of my own generals I'm able to upgrade title with. For example, I can upgrade Belisarius but I can't upgrade Lupicinus title. If I come across a reason as I play, I'll be sure to come and update this thread,  but for now all I can say is that it may be dependent upon some weird usage requirement or something, as I don't really use Lupicinus but I do use Belisarius quite often. It may also be due to a maximum title that a general can achieve, but again I'll try to find out more as I play.

2. A list of titles... there may be one available somewhere on the forums here but just in case there isn't one I'll list them here
Civilian: DMG Increase 1%, DMG Reduction 5% (to shorten things I'll refer to Increase as INC and Reduction as RED from here on out, I'll also cut out DMG)
Gentleman: Inc. 2%,Red. 6%
Squire:  Inc 3%, Red. 7%
Knight: Inc 4%, Red 8%
Baronet: Inc 5%, Red 9%
Baron: Inc 6%, Red 10%
Viscount: Inc 7%, Red 11%
Earl: Inc 8%, Red 12%
Marquis: Inc 9%, Red 13%
Duke: Inc 10%, Red 14%
Prince:Inc 11%, Red 15%
Crowned Prince: Inc 12%, Red 16%
Royal Duke: Inc 13%, Red 17%
King: Inc 14%, Red 18%
Emperor: Inc 15%, Red 20%

You also mentioned abilities in this question, and I'm really hoping you mean the Inc and Red that I mentioned, if however you mean the General's abilities, I believe those are unlocked through ranks as opposed to titles.

3. If I understand correctly, the personal items only effect the general they are assigned to. Using Attila, at level 3 his bow will give him leadership level +1, and I believe that applies only to Attila.

4. From what I can tell, mercenaries only act as a general's personal army. There may be a feature that allows you to recruit them within conquest or campaign but if that feature exists, I have yet to discover it. Again, if I find information to the contrary I will be sure to add it here.

5. Ok I know I initially said I would answer Charles question and then snowman's question, but upon review I believe this is asking the same thing in both cases. Yes, you absolutely can replace a generals unit once it has been chosen. I remember that I too was infuriated at first because I thought you could not replace a generals unit, but I tried again a little later and discovered I could. I'm not sure what changed that allowed the replacement but I will share a couple hypotheses that you can test. I will also share how to replace the unit, because it could be that you are able to and just don't know how.
 Hypothesis 1: There may be a campaign requirement that you have to complete before you are able to replace a generals unit, but that seems a little far fetched, however in my case I'm pretty sure the next hypothesis was false hence why I came up with this one.
Hypothesis 2: You may have to have another mercenary unit available to replace the one you are removing. Again, I'm not sure this is the case because if I'm remembering correctly I had other mercenary units available when I tried to replace the generals unit but couldn't.
Finally, HOW you actually replace the unit. First, click on Headquarters and find the general who has the unit you want to replace. Then, click on the picture of the current unit that can be seen in the HQ. This is important: DO NOT click on the general themself, or you will be taken to the generals screen, not the unit screen. Once in the screen where you see the general on the left and their units on the right, click dismiss below the unit of your choice. A confirmation screen will pop up, and once you have confirmed, the unit will be gone. Now, you can recruit any unit you want within that slot, provided you have the gold for it. However, please keep in mind you will not receive any gold back for dismissing a unit, so it may be better to wait until you have extra gold to attempt replacing a unit. Also some extra detail for snowman's question, one draw of a unit is not enough to recruit the unit. For most units, you need to draw them 30 times for the initial recruitment and many more for upgrades.

6. This is a question I can only take a stab at. I will once again default to Lupicinus as an example because he is the only general that I know you have unlocked that I also have unlocked and can view to try to help. When I click on Lupicinus, I see that he has an ability that is locked, namely his "leadership" ability. I believe that upon ranking him to Commander 1, he will unlock his 4th skill. However, I currently do not have enough heraldry available to rank him up to Brigade Commander 1, which is where it says "Unlock 5 skill slots", so I'm not entirely sure what happens when you gain more ability slots than the amount of abilities the general has, but I will try to grind out some heraldry as soon as I can to try to answer this question for you.

As mentioned in my last responses, if you come across any further questions feel free to ask and I'll try my best to answer them, but unfortunately I am a student and don't have quite as much time to play as would like, so chances are as time passes you'll have actually progressed further in the game then I have, but until that time comes, as always, happy conquering! 

Yeah, thanks a lot. That’s extremely helpful. I suspected hypothesis #2, but am glad you’ve verified one can eventually can dismiss. I can now add Legions with confidence that I can dismiss and upgrade later. I’ll try to keep tabs on when I can dismiss and see if I can verify the conditions and get back to you and the board.

1. Resetting the general's capabilities resets it to the point of recruitment while not affecting rank or title of the general. In essence, this allows you to just change where you assigned the generals capability points. While I have never found a use for the feature, I suppose it could be helpful if you find yourself in need of a highly specialized general. 

2. In my experience, I have never run out of gold when applying units to generals. Obviously, we do not have the same amount of gold so I can't necessarily speak to that but I can say that the most gold you will need to recruit a unit to a generals army is 20,000. While gold rewards vary from mission to mission, you should be gaining back enough gold that you can put very good units on your generals. If you are worried about it, I would recommend spending gold first on your best generals, and then assigning weaker units to weaker generals. If you're ever short on gold though, play through a couple conquest missions or campaign missions and you should be back up pretty good.

3. Many generals have a chapter requirement before you are allowed to recruit them, so obviously you have to weigh whether or not you can beat missions up to the point where you can get the desired general without different generals or not. However, which generals you decide to recruit should be highly variable upon where you determine your weaknesses to be. I personally have a lot of calvary and I lack in infantry and range, so I try to spend my medals on generals specializing in infantry and range to make up for that lack. I also like to first upgrade existing generals before getting new generals, in my experience it is cheaper and more efficient, but ultimately, it is up to you.

As always, happy conquering.

Also, I'm happy to answer whatever questions you have, it gives me a nice break from studying and I love doing some research to try to help, so please if you have anything else don't hesitate to ask, it's fun to help! 

How exactly do bonds of generals work? Do all of them get the same bonus or what? Should be all of them deployed to make this bonus active or is it enough just to have another general in our Headquarter?
And, finally, is it worth it at all?

You need to deploy them for them to take effect. I only have one, Justinian + Theodora, so I can only speak for that. The extra iron lets me sometimes spawn a better unit than if I didn’t have that boost (and the silver boost would probably do the same), but the really gamechanging one is the +2 technology. You usually only get one per turn. that lets you deploy war gear much more efficiently (important for navy, navy hits really hard). Of course, I have to spend one slot and unit on Theodora when she’s not that powerful, so I don’t always do it, but atleast she has a handy active.