SD Rhyperior vs SD Tyra

SD tyranitar beats SD rhyperior in everyway when at the same level. So I would say yes it is still a very worth it trade.

the trade off is rhyhorn candy might be easier to come by if you are lucky with getting a rhyhorn nest, but over all, it'd still say Tyranitar's the superior option

That's an easy one. You can get as many SD Rhyperior as you want going forward. You can't get another SD TTar unless they do some weird repeat of the Larvitar CD, or something similar, like a Ttar raid day where Ttar comes with SD and Foul Play (or whatever). Further, Rhyperior isn't quite as good as Ttar, although Rhy will stand up to fighting better (not so with Grass or Water, though).

I'd take the trade in a heartbeat, unless someone else wants to offer you a good legendary.

Legacy movesets always have some value at least as collectibles...but a legacy moveset that makes a mon top of the DPS charts of its type? Definitively valuable!!! More than a shiny i would say (since it's usefulness over aesthetics). People went nuts to obtain shadow claw gengar before lick was avaible since it was a top ghost attacker. Same with Meteor Mash Metagross if we get some more fairy raid bosses in the long run!

You can, but depending on how many lucky you have so far you might want to prioritize a shadow ball Mewtwo instead.

Smack down Ttar is better than rhyperior, but will do less Dps than rampardos when he is released. However much better than tdo there still.

Others already answered, just make sure the mon you trade is from Jul or Aug 2016.