Some bizarre bullshit happens when I open inventory with makeEventsSingletons=true and these errors have no connections with each other other than that it only happens with this options enabled and I have no clue what causes them.

Some of the logs:
debug-3.log.gz
Crash reports (I've collected a fuck ton of them):
crash-reports.zip

Seems like something happens with capabilities but I am not sure if that's the real reason behind this

Also if you would need more I can make a bunch of them with another debug.log file, just ask me.

I had a quick scan through, are the crashes pretty much all of type Duplicate Capability Key?

I had a quick scan through, are the crashes pretty much all of type Duplicate Capability Key?

yep, pretty much that it is

Should I try narrow down the issue to a more concrete set of mods and configs?

If you have time sure, but I'll probably approach this differently in my reimplementation