Good write-up! I agree that the M-Network 2600 releases were quite good overall; I can even appreciate the merits of 2600 BurgerTime, despite its obvious limitations. 20th Century Fox is another underrated third-party publisher: they produced almost the same number of 2600 titles as Mattel, with roughly the same hit-to-miss ratio (at least in my opinion). Anyone looking for something new for their 2600 beyond the usual Atari/Activision/Imagic trifecta would do well to start with either of these companies' libraries.
Not 20 minutes ago, I posted an entry on my blog about M-Network games, which I thought were generally quite good (although I know that is not a universally held opinion). http://cynicaster.bl...-m-network.html
(And yes, the unusual shape of the M-Network cartridges was a cost-cutting measure. Plastic tooling is quite expensive, so instead of tooling new cartridge cases for each system they intended to develop for, Mattel simply used their Intellivision shells and designed simple snap-on "adapters" for each target system.)