Thanks for the details. The revised chassis layout for the 2 Stroke that is floating around the internet shows the circuit with one power tube and clears up the vagueness of the shielded cable connection at pin 2 of V1.
The Weber Maggie kit is the same amp, you can see it at the Weber site,
it addresses the “boost” switch pop, if your tech did not already fix it.

Regarding the grounding issues, it’s true that single ended designs can be noisy but with proper lead dress and grounding the 2 Stroke can be quiet (both of mine are wired pretty much like the original layout and are VERY quiet). Unless there were other issues not mentioned, it looks like your tech’s grounding changes were personal preference. The 8,16 and 40uF caps all tie to the same ground. Using one of the transformer mounting lugs as a ground point works too.

The shielded cable cable is there to reduce noise and interference, you are correct that it is possible to not included it (Fender never did) but it’s still a good idea. Using a shielded cable with one end (only) grounded, can really help quiet a circuit in some cases.

Good luck with your amp.