IIRC MPPs are quite frequent for a while. But TBH I never looked into it closely.

Yes, I mean decoys. You do have to trust them. The only difference is what number of interactions you need to have sufficient confidence that there is a sufficient number of honest participants.

Even with accumulators you're still trusting. The only difference is that the parameter becomes all users which is really good and efficient but effectively homomorphic with just using everything as decoy or using every single LN node in path.

As long as you believe that for some number of participants it's safe enough it's OK.

I heard Coinomi suggested to newbies because it had a bunch of other shitcoins as well as btc.

LN doesn't require scanning comparable with Monero. It only requires block filters which are comparatively smaller and only for channel states, not for every transaction. That's orders of magnitude difference.

In LN broadcast privacy is only an issue for initiators and only if they don't use Tor, which is trivial to enable these days.

More people increase anonset but make scanning more expensive pushing people towards revealing their view keys to third parties. There are also obscure probabilistic attacks that are only possible because anonset is high. But once FCMP is no longer vaporware they indeed cease to exist.