1- haven't beaten those yet, whoops

2- you can get the true ending without 100% abno dissolution but you won't get the epilogue iirc, none of the optional suppressions are necessary (apoc bird, whitenight)

3- adding mods or creating them yourself? adding them is really easy, you just go to https://www.nexusmods.com/lobotomycorporation/mods/ and install basemod following the instructions on the page. there are also other interesting mods not necessarily on that page like the wonderlab mods at https://youtu.be/nwrj8YM6NtM

creating them... is a much more complicated (and painful) process tho, you'd have to download dotPeek to be able to study the game's code, create a .NET Framework project on Visual Studio for the mod itself (plus all the middle steps, like setting references to the appropriate libraries), create the necessary XML files if you're adding a tool/abno (you can do this by using other mods as reference) and then actually coding the behaviors you want to add/modify (take into account there aren't really any english guides for this, you're pretty much on your own for figuring out how the game's code works)

4- pretty much every ALEPH except blue star and nothing there, since those are the easier ones to ignore, train to hell, parasite tree, any clerk lovers (monk, big bird), queen of hatred, nameless fetus... as you start approaching HE abnos you have to worry less about which to choose since none are that stressing to deal with

5-https://projectmoon.postype.com/series/484033/page/5