I think making them immune to a core game mechanic is a bad way to go.
With the LMG42 long range DPS decreased (normalized to the level of other weapons), I think the way to go for now should be a decrease in their build time. This means Penals can finally fulfill their long range DPS role, which they can keep up for a long time from cover and pretty cheaply when Conscripts merge into them.
One main reason for their lacking performance (being beaten at long range DPS) is now gone. This means that the meta might shift into a more penal-friendly state all by itself.
The decreased build time would still be good so the massive hole in field presence created by long build times of T1 + penals is decreased a bit and to incentivize people to try them out again.
Good analysis and good suggestions. I'd add that the introduction of tier0 Ostheer units really screwed penal builds in my opinion, mainly because of the build times.