But Mewtwo is both fast and bulky; anyway, if I opt for Deoxys-S DS, is Shadow Ball the attacking move of choice?
Also, must the standard Deoxys-S lead carry Spikes? I mean, it alrady struggles on more than one turn against ScarfRai, and Psycho Boost is very useful to revenge-kill Gengar and Heracross...
It's fast, but not fast enough on lead spot. Deoxys-S works fine as I used it few times as a Double screener. There are three options to use as a last move - Taunt, SuperPower, Shadow Ball. Also loosing automatically against Deoxys, Deo-A and Deo-S leads isn't that great.
Taunt keeps any entry hazard from battlefield with Deo-S speed it's perfect. Forretress has free turns to throw entry hazards and it's surprisingly often on lead spot. MewTwo also may run, but much lower speed makes a difference here. SuperPower is mainly for Dialga and Tyranitar leads and if you want to beat Darkrai leads 1on1, you may run Lum Berry + SuperPower, however I don't recommend it. Shadow Ball is for other Deoxys forms. Your choice ;).
Never, but never use Psycho Boost. Too many pokemon resist it and unlike Deoxys-A power it has on Deoxys-S is crappy, don't even bother with it. And Heracross + Gengar aren't that often to even use it just for them.
Anyway, is Selfdestruct vital to ScarfTwo? Is Magic Coat a good idea to bump back Dark Void? Should I ever use Psychic, and if not, why did it even make it to other option?
Selfdestruct isn't bad on Scarfer, so you may use it, if you really need it or your Blissey weakness is massive. I don't recommend Magic Coat as Darkrai will switch out anyway, as many MewTwo leads are scarfed and won't risk taking Focus Blast. MewTwo also survives Dark Pulse from ScarfRai, so it's another reason not to use it. Imagine - you use Magic Coat and... Scizor, Tyranitar or Metagross switches in. Your MewTwo is dead when they use Pursuit.
Psychic is meh in Ubers because so many pokemon resist it. Too many to even bother with it. Also it gives too many free turns for stuff like Darkrai, Tyranitar or Scizor. So no, never.
Also, if I want Mewtwo to have a Fire-type move, Flamethrower or Fire Blast?
To be honest I prefer Fire Blast for one small reason - Scizor with Rain survives any Flamethrower under rain with bulky EVs. And most Scizors in Ubers are defensive. Fire Blast OHKOes most of them, even in Rain and only the bulkiest 248 HP/252 SpD doesn't die from Fire Blast under Rain.
I hope that helps ;).