What I see on the M1 are pure T2 details. The front and rear cover are different between T1 and T2 and the scribed lines between them are different. The T1 are angled in towards the rear, so is not parallel with the rectangular panel next to it. Also, the T1 has the indented hole in the front cover as well as a small dimple at the edge towards the front, whereas the T2 has it filled in and the dimple rotated slightly to the side. That is one of the features you can tell T1 and T2 skulls apart. The T2 production pictures with the actual endoskeletons used for the movie all have the T2 front and rear cover, with it looking like the rear cover is part of the casting and not separate as on T1 - the 2 T1 style you see in T2 is the fallen over endoskeleton that gets shot by a resistance soldier, as you see the endoskeleton with the indented front cover in an auction video on youtube, as well as the one coming out of the fire at the end of the title sequence. From all other I've seen of T2 endos, they have the T2 details.
Another T1 is seen in the T2 teaser trailer as they hadn't made new endoskeletons at that point.
Icons may or may not have molded the M1 temples. But I agree with you that they did not mold the whole M1 for their skull. As you say, details differ.
Timeless has the T1 style front cover and the angled scribed lines back to the rear cover and the rear cover is rotated compared to all other T2 style castings, indicating it was added to a casting that had no cover.
LFS, as I have been looking very close to theirs, there are many alterations. Areas sanded down, details changed, other parts of the endoskeleton completely fabricated from scratch that doesn't match what we see in SWS castings. Close, but not exact. Likely done as a standard to keep certain things obscure, as has been the norm in licensed prop replication over the years, or because some parts were difficult to mold and proved easier to just scratchbuild as close as possible.
Did this up roughly to show the difference between what I call T1 and T2 style. Used a ICONS and Timeless piece to show what I mean. These are consistent with what I've seen from T1 and T2. Though, the Timeless indentation at the front cover is not similar to T1 castings. They altered it.