It’s a little hard to see at this angle, but I think they’ve got a few inches of space between the bottom of the trunk and the bottom of the cvac. OP, find a way to fasten the top of your wall to the joists. It’ll be far better than relying on the blue 2x4. It’ll probably involve cutting the cvac in a few places and gluing it back together with couplings.

It costs you $0 to ask, and you could potentially learn something very helpful. Definitely give it a go

If you’re cutting steel/iron, you should be using an aluminium oxide wheel. If you’re certain that you’ve got the correct abrasive, throw it away and buy a different brand. Defective wheels aren’t something you take chances with, even with proper PPE

The keys would’ve been either bar-and-bit or barrel-and-bit

Knob and tube wiring? That’s a relic