I’ve got a CX bike which is QR all round, perish the thought I hear you say, the wheels will fall out Guv, it can’t be stiff etc etc.

But in all seriousness, its QR135 rear and has mis-matching QR’s will a 130mm qr be ok, or does it have to be a 135?

I wouldn’t and I’m not sure many will say they would. Imo a pair of Shimano qr are cheap and as good as any if not better.

So long as the thread on the QR axle goes all the way through the threaded bit of the nut it’ll be fine.

(in most cases for max strength, a thread needs to be engaged by its own diameter into a nut – eg a 5mm bolt needs a 5mm deep nut. If you’re dealing with an alloy nut on the end of the steel QR axle, you’ll want a bit more penetration to compensate for the weaker material)

10mm engagement in aluminium.
https://www.nord-lock.com/insights/bolting-tips/2015/the-experts-thread-engagement-in-a-tapped-hole/