The early Riddell plastic was called 'Tenite' which not only yellowed with time as you stated but it was also quite flexible and in fact Riddell sold a "shoe tree-like" object to help them keep their shape when not in use. Riddell soon switched to a stronger/firmer plastic called Kralite but it was no longer a clear shell instead it was an impregnated (colored) plastic.Boy, does it ever. Thanks, Jeff!
Marietta and MacGregor did not use tenite plastic. Mac used a plastic called Merlon for their 100MH model clear shell and I believe Marietta used a plastic called Lexan for their clear shell. Your are also correct in that Bill Kelley and his partners bought out rights to the MacGregor helmets but it was around 1974/5 and relauched in about 1975/76. BTW, part of the reason Mac sold out to Kelley was because they had a terrible time filling the orders and maintaining the warranties on their 100MH model as it often cracked. Bill Kelley strengthened the edges of the shell on the 100MH and had less cracking problems but did not eliminate them completely.
Marietta, which as you said was a much thicker shell and didn't have the same fragility to them as the Mac/Kelley but they were extremely heavy (especially compared to canvas suspension helmets).
Finally, in regards to Riddell's purchase of MaxPro I believe that the Riddell XL shell used for their (now outdated) VSR 4 helmets, which is still worn most famously by Tom Brady, is in fact a MaxPro/Kelley shell.
BTW, in response to your initial question about the Packers helmet I would agree that the time line is accurate (Riddell introduced the Kralite plastic in 1954 and stopped making the Tenite in 1953) so it is definitely possible but I have no hard evidence. Although that evidence should be relatively easy to find as the clear shells appear to have what I call an 'effervescence' to them, (especially compared to impregnated or painted helmets) in the photos for both B&W and color.
Hope that helps.
I know less about this period than most in the Packers' history, so help is especially welcome with this one.
I'll see what photographic evidence I can find from the period, 'effervescent' or otherwise. And if there's anyone else here who can help, you know what to do.