Since nobody has tried to answer, I'll do my best.
I don't know if it has stabilized, but seems to be working well, so unless anyone complains, I dont think they'll change it.
No, you cannot use FinQWERTY layouts intended for stock with Lineage. Some keys are mapped in hardware different than stock, i.e.
physical: stock/lineage/sailfishos
F (fxtec logo): Home / Meta / Super
Sym: Unused / Right-Alt / Slash
Slantarrows: something / weird nothing / right-alt
Ctrl: Right Control / Left Control / forgot to check
Esc: BACK / Esc / Esc
So to use a finqw