Yes, although Doom isn't categorized as a real 3D engine as such.

Yes

A few years ago I seriously contemplated making a game using software rendering, in part because the only target was SDL. I actually got a 3D engine running 2K@60Hz with HL2-level scenes, with the CPU basically idle, so it's not a terrible idea.

(Then LLMs started taking off and I decided I needed to reskill out of the industry, just in case)