1 - 50k xp per MP
2, 5 - Each stat maxes at Lv3 but you can only invest up to total of 20 MP per hero so choose wisely (Priority: DATA > Atk L + Burst + Crit > Atk M > Everything else)
3 - 1 MP per upgrade. Non-refundable even if you cancel already invested stats.
4 - ...???
6 - Yes