[bbz]

question 1: yes but if the armies are small they might miss each other and just conquer the enemy's territory. On the other hand its quite possible for the commander to stop the advancing army(that's why commander blocking is considered an exploit in most games since you can use a scout and block a big army's movement for a long time)

question 2 no he cannot stop them in that case. Only if hes going straight against them.

question 3 No it wont stop the army from moving. But IIRC if you cast a magic attack ritual(horde from hell for example) the commander will join and make the attack in the magical phase of battle.
Otherwise the battle will occur after the troops of person B have moved away.

P.s Also most of the quesions people ask can be tested for 2 minutes in the dominions SP game. There is a really good tool as well which helps you answer the tougher ones.(Debug mod) It drops the price for wish to 1 gem, and you can summon a commander with 9power in all magic paths. And with him and with infinite gems you can test a lot of things.

An extra info on magic attack rituals: If you have 3 province A,B and C and use army with map move of 2 to get from A to C(assuming that requires 2 map move) If your opponent uses a magic attack on province B(and takes control of B) your army wont be able to move in C and will stay at A.