I get pumped more quickly than usual and power out real quick, then my forearms get completely cramped up (this hurts a lot!).
Have you investigated pain transferral from your shoulders? Or radial nerve impingment?
I've had it on and off for 20 years. I can do a route and lower off feeling so wasted I can't even untie, but there's hardly a vein showing in my forearms, I think it might just be they've never really developed much.
I'm not a doctor, but read too much on the internet in search of information about my next injury before it happens. Google search Exertional Compartment Syndrome Forearm Climbing or some permutation of these terms and see if it fits?
My left arm plays up Luke, as I've mentioned before. Non of my mates really get what I'm on about. Physio mentioned compartment syndrome, particularly as I get bruising sometimes but from everything I've read, I'm pretty sure that's not applicable to me.
I have a decent ability to produce lactate but no ability to use it in climbing..
One big thing that I think has changed this for me, apart from sleeping flat with arms straight, is that I do a lot more shoulder stability / chest opening exercises - I.e. 20-40 mins twice a week.
With cramps etc. I would also have a blood test to check minerals levels.
As for the rest, the pins and numbness definitely seem ulnar nerve impingment. As said, I noticed great improvement from sleeping with straight arms, instead of with bent elbows with all my weight on them.