Happy ending to original problem
OK, so I finally got out to the range again with the LW carbine - freshly cleaned & lubed with Slip 2000 EWL - and 10 Pmags pre-loaded from my newly arrived case of PMC X-Tac green tip.
Flawless.
Also the Slip seems to have not cooked off to any great degree, since everything still looks wet.
Finally, although the X-Tac has green paint on the bullets & red primer seal, there isn't green & red confetti all over the action like with the PPU (even after 300 rounds).
So, looks like the LW carbine is now also GTG.
Thanks to everybody who contributed to this outcome :D