While on the topic of reinforcements from Harad : I seem to remember that in 3.8 upgraded Mumaks had this really cool warpaint. Did this get removed in 4.0, or does it come when the Mumak is at max level ? (I never got a max level Mumak, because I don't take care of them well enough ^^).
Mmh, as for the archers on the Mumak's back, if spawning the beast with them on top causes the bug, wouldn't it be possible to give the Mumak a power/upgrade that unlocks (either for a cost, or automatically) at a chosen level and mans it with archers ?
Another possibility would be to have a hero from Harad (much like the one in 3.8 that alowed the building of the Mumak production building) (you'd get him through reinforcements or a power) who would be dedicated to supporting Harad troops, and would have as a power the ability to select a Mumak and man him with archers : ).