Power up a Scizor with FC/XS?

Definitely, since it's 100%, and also useful for Exeggutor and other psychics.
Actually, any IV above 90% and 12 attack would do.

With it being 100% and fun to use I say yes. Though from what I read it won't be too great in Mewtwo raids (too squishy).

Put that dust into Tyranitar instead.

I did power up one and it works great for exeggutor raids. Lasts long enough to bring down eggy's life bar close to half. It won't be the same with mewtwo. But my policy about usable 100% is "juice them up".

Yes power it up. But don't stop at level 30; take it up to at least level 31.5, where fury cutter starts doing 4 damage instead of 3 against Mewtwo boss..

Yes, it is a supreme celebi counter as well and very good against mew.

Depends on Stardust. Personally, I'm raising up a 91IV (with A15/S15) Scizor with FC/XS up as high as I can go with it as I walk a Larvitar to get enough Larvitar candies to raise my Ttar's up as high as they go. Given how difficult it is to get candies for Larvitar's without using Rare Candies (which I've done in the past, and may do in the future - it depends on whether I get a good Mewtwo or not), I've got LOTS of Stardust that won't be used for my Ttar's, so I'm putting that into my Scizors for now.

If I had all the candies I need for Ttars? Ttar all the way. It's just too useful, particularly in the current Raid Battles where Fighting moves are quite rare.