It would be illegal if I plugged a y-split into a power expander, and y-split from its output, right?

You are allowed to plug a y-cable into a power expander, but you can’t “Y off a Y”.

No, that is legal (or was legal in Starstruck). See this Q&A.

This is probably something I should know by now, but how do old Q&A rulings that still apply to current games work? Is it safe to assume that they are still correct interpretations of the rules unless otherwise specified?

You can, just as long as you end up with 12 or fewer motors, and don’t continuously Y split off of other Y splits to create more ports.

That’s a great question for the official q and a!