Indeed, there is one Super Star Destroyer (commanded by a mysterious unamed commander) left...and a bunch of regular Star Destroyers.

Same.

Or it could be that their Son (Kylo Ren) has gone fully bad?

Re: Resistance Megaweapon....

Actually, the Force Ghost thing is explained and handled REALLY well in S6 of TCW when Yoda learns it all. It’s quite poignantly done even. Like how you and I are lifeforms that share the building blocks of the same stuff that makes up stars...the Force is like that, and what Jedi’s are learning is how to manipulate…