That's a good feeling when you can actually find a problem and fix it right away! Those Volvo spark plugs also make a big difference.

the improvement when MAF is disconnected shows its less likely to be drivetrain.
What about a simple smoke test to eliminate vacuum, vac lines are only one source of possible leaks.
A jar, a cigar and some hose will do it. Lots of DIY video on youtube.

Isn’t that frustrating, 15k on a new part?

Compression braking on a downhill is usually valve seals or valve guides.

Piston rings as well, it sucks oil mist from crankcase.

Thansk, vtl, really interesting! Do you think that my smoking is probably piston rings rather than stem seals or valve guides then, if it only occurs after long periods of high vacuum / engine braking, rather than after idling or upon start up? Likely, both. It is a good idea to replace valve stem ...