I played the EFF out of this game on release in the west.. is it alive? Worth coming back to?

Is it alive?
Yes but it's dying - According to steamcharts the amount of players has been dropping every month and currently the average is about 25k - this includes the bots (yes, they still exist)

Worth coming back to?
Unless you have a few friends or guildmates you can play with, I would say no

buying gems and other tradable's if you're quitting.

I have one of those platinum skin chests, with the weapon and armor... it's tradeable, from when I bought the platinum set when it first got released in the west.

I came back a week ago and haven't played in a year. They are giving bonuses for returning player to catch up, been having lots of fun and learning Legion raids. I'll probably play a couple weeks/months until I get hardstuck or bored.

what do plat skin set / grim skin set go for atm?

With friends yes, it's worth getting back into

Without friends no, it's not worth getting back into.

If you're returning, then prepare yourself for major gate keeping as you get higher. Imo it's probably the worst part about this game to be honest.