No, you can't install any 3rd party ManifestV2 extensions in Brave. Neither are they shipping any changes to the browser engine that Google doesn't maintain.

E.g. they tried to implement dark mode website conversion and decided it's too hard to do anything that Google themselves don't do.

yes, but only for those extensions that the brave team maintains because all other v2 extensions are likely no longer maintained at all: https://brave.com/blog/brave-shields-manifest-v3/

so in general the problem is not with supporting v2, the problem is that except for a few special extensions that need v2 features there is no point because all those v2 extensions out there will either be ported to v3 or they will be unmaintained.

the maintainers of chrome forks with v2 support lost interest because the developers of v2 extensions stopped maintaining them.