Lol. You and your ally are going to experience the same “time” perspective. No relativity in effect here. Nor “bullet time”. The replay speed is what was different - you can watch a replay at any speed.
Btw, in the replay overlay you watch the game at the speed specified in the overlay. You were probably watching the replay at “Faster” speed. I don’t think there’s a way to determine what the actual game speed was corresponding to your game, except knowing what the actual game speed was and then setting the replay to that speed.
As far as I know, Casual is the same speed as Normal. But I’ve not been matched with a casual in a while so can’t recall how slow it was.
Actually, it’s the exact opposite. If you’re in a party, the difficulty, and hence the game speed, is selected by the party leader and you both have the same diff & speed. To get matched with a player with a different difficulty, it would have to be random.
(The in-game menu difficulty sometimes bugs out for a party after the first game and reverts to the one previously selected by the previous party leader if you then changed leaders before the first game.)
No, you both get the XP according to the higher difficulty player. I’m 100% sure of this as it’s been happening more frequently when prestige CO’s at a low-level on Normal (until I reach a certain power spike) but I get matched with a player on Hard - I get the Hard XP, not Normal. It’s the game stats/achievements which counts it differently.