Yes, all that Robert says is 100% correct.
1. Can Large Forces keep spawning units throughout a particular rounds sequence? Or do they just spawn one unit per any particular rounds sequence?
As Robert said, yes if the activation checks say so.
2. Mosby enters a space with an inactive unit. A roll activates it. The Union gets initiative. Now: do all the Union units get to move before combat (thereby possibly making a stronger joint combat against Mosby) or does the Union unit in Mosby's space have to attack him immediately?
IIRC, first Union movements, then combat. And Robert is correct again giving the advice about having the Initiative.
3. Mosby gets "3 points" for a Union Strength Point Loss. Is that 3 points per point of loss? Or 3 points per any single instance of loss? Example: Mosby combats and kills a 4-strength unit. Does Mosby get 3 points or 12 points?
Yes, it is 3 Performance points per Union Strength Point Loss plus 5 for victory. In the rules it is in page 9, second column: Combat Results - Retreat.