Hellgate modding is hard business, no doubt about it. Only patience will get you to the goal; if not today, then tomorrow.
1. How to mod an already existing mod? Reanimator does not want to open such a file. He only wants the HG originals. If I change the value here to 5000 and put it in another mod, both mods will have Items.txt files and it won't work.
You'd need to merge the changes of the two mods, either manually (by re-entering both mods' edited entries into a clean items.txt) or through some other way. Optional mod components would be where I'd look for the latter option, as it sounds close enough to your goal here. Though I haven't explored this area at all, myself.
2. Is it possible to set the achievements in advance after entering the game? So that when you enter the game you get + xCC for PvP achievements.
Not that I know of, but you can change the achievement requirements into something that's actually doable within the game. You'll need achievements.txt for that, to change these achievements' "type" column into something other than PvP wins. For instance, you could make them a "kill" type, change the "complete number" field to "100", and set the "monster unit type" to "undead"; now you get the achievement skill after 100 necro kills.
3. Yesterday half a day of running after the Balbi Ring is a big misunderstanding. Bryan tell me if you know how change the quest "A Leg Up" it's index 109, it's also 108 but not repetable so isn't it to get an UNIDENTIFIED BALBI RING? Instead of the award (offerReward_string), instead of the name, there is simply Wart Pegs Leg, there is some Quest_Joeldaily ... so again look for something that is and change the type of reward there. I know it's a cheat but I have my patience limit. I'm tired.
Here you'd need to track down the repeatable quest's reward in offer.txt too; "treasure 0" looks like it can reference the desired reward.
Balbi's being unidentified should ideally not matter, as any Balbi's spawned naturally should roll for random affixes. But if you'd like to change that too, offer.txt also has a "do not identify" column you can set to "1".
4. You don't actually need an editor to change the boss in the catacombs. The problem is that the boss is not called, for example, Occulis, but I don't remember .... pm_15 or something like that.
That's true, bosses' internal names very often don't match their in-game names. Look through strings_monsters.xls for internal names.
5. Baracelet, necklace I have no idea how to set it up for sale in a shop like a premium shop used to be.
On other characters I saw + 5AA or + 7AA. In the guides in the premium shop there were + 10AA and on the unique items + 20AA, so I do not know which ones are legal and how much it cost at Global.
Setting up a dedicated shop from scratch shouldn't be easy, and my ideas on that front are very limited. I suppose you could add them to regular vendors (though vendor wares' location escapes me now), or as recipe results, or as boss drops. To me, boss drops sound the most logical - but that's just how I'd do it.
For +AA values, I'm afraid I can't remember. The wiki lists +5 too, but I think it used to be higher - probably based on level. To change that, you can track down the affix they use in affixes.txt, and modify its values there.
Last second. I changed the stacksize to 5000 in Adrenaline, Baisc and Advanced Fragments and I did it in a separate mod so that they would not exclude each other and still in the Adrenaline pack I can only 10 and 100 in fragments. Wtf?
No idea, I'm afraid; I'd need to see the data to try to pinpoint the issue. Though it could just be the items have hard-coded caps, in which case fiddling with data won't bypass them.
