A movement speed penalty would be fair and maybe the crew not being able to pop out. But I'd only support the latter if you got rid of all the other crazy requirements. If it's going to cost munitions, a fuel upgrade, and veterancy, and be limited to one, it had better be firing nuclear missiles. I think the best thing to do would just have it be an alternative purchase from the .50 cal upgrade, and cost 150-200 munitions. If you get lower speed and can't pop out the crew, I don't think people would spam them so much.
But in all honesty, I'm now just thinking the best solution would be to have the Calliope as its own unit, but still have a normal main gun that's a lot weaker than the standard Sherman's. Around the strength of the Panzer IV Command Tank (albeit a little weaker against infantry). I think that would make American artillery a lot more flexible, mobile, and unique to the faction, rather than making it like any other artillery-only platform.
I think it would be really interesting if it could actually fire on the move like the Scott, but have higher scatter while doing so.
I doubt it, you have a Sherman that can fire it's main gun and fire rockets. I think they will be spammed if you don't put up these requirements..
I agree with your second section, but the idea of purchasable upgrades gives so much room for diversion