I would almost certainly buy an HG-70 hyperglide cassette if I switched to a freehub. This Regina is built like a tank. The thing must weigh almost 2lb alone!!

What you're describing with the two freewheel cogs exists. White Industries makes something called the "Dos" which is essentially an expensive version of what you want.

Pretty sure there's no harm in wedging a 130mm wheel into my 126 frame. I've done it before with a crappier bike. Chain clearance depends a lot on the frame being used.