I'm not an expert by any means, but I'll have a go until a real expert chips in...
1: Special tasks may happen with new events, or repeats of old ones, but if you've done a special, that's it. I think I've done about a dozen.
2: Wait for events which those Pokemon are a part of, check eggs for potential pokemon, remote raids, or go hunting.
3: You won't lose the Pokedex entry by deleting a pokemon.
4: Remote raids. Events. Be a higher level yourself to see higher level pokemon in the wild.
5: IDK anything about that.
6: I think so, yes, because a shadow pokemon gets a bonus to stats. But you'll probably want to get rid of "frustration" first, which is only available at very infrequent times as far as I can tell. Also, shadows are cheaper to level up.
7: In theory I think so, yes, because apparently attack contributes more to CP than def/hp, but I haven't bothered to min/max that. I just go with 15/15/15 or as close as I can get. Matching the weather type seems to help me.