Plexamp can do all of this (not sure about mdna speakers though) and it has Tidal integration.

Finamp will also work but it lacks library/genre radio and mix support last time I checked.

Should be the same across any instance.

Hot: calculated using the current score (up/down votes) in combination with how new it is.

https://github.com/LemmyNet/lemmy/blob/main/migrations/2023-10-23-184941_hot_rank_greatest_fix/up.sql

Active: calculated using current score in combination with the timestamp of the newest comment:

https://github.com/LemmyNet/lemmy/blob/main/crates/db_schema/src/aggregates/post_aggregates.rs

I think in the library settings there’s a box to ‘prefer local metadata’. That’s should stop Plex from changing artwork.