There is no such thing in this game system as periscope ATGM vehicles (T0W in elevator, JPZ-HOT), nor for detatched controllers from ATGM vehicles firing from defilade (striker, BRDM-Malutka etc). Nor any concept of "turret down" position.
No. Reducing size over the M150 would benefit it too much when caught out in the open. In fact it would be more noticeable in the open with that kit on top, so having it the same size as the M150 is probably giving it a litle bit of a benefit anyway .
I would agree, I don't think there's a way to represent with the game engine without having additional issues.
Also, what cover would it be hiding behind that its not taking body hits from most weapons as it is? The way Steel Panthers does LOS you're definitly not getting to see over hills or anything with the M901, so unless you're behind a giant berm or in an entrenched position, I don't see most large autocannons and definitly tank guns being able to smash through whatever its hiding behind. Personally I think that with the M901's 40 vision which gives it the ability to sit inside dense forest and still punch out missiles at targets, and in most cases remain hidden unless the vehicle also has 40+ vision is a pretty good ambush advantage.