I didn't realize this couldn't be done in Chronicles! I've done it in multiplayer over Fightcade previously and I just assumed it worked in CoM as well...
As for difficulty differences; the only clear difference I ever saw between arcade level 8 and CoM Master difficulty were the amount of skeletons the shadow elf has during the refight in Sable Tower (Master mode had fewer if I remember correctly). Hard to align the difficulties when Master mode isn't even difficulty-8. I really wish they were 1:1!
On CoM Apprentice, Lich summons two zombies per Animate Undead. Arcade default (level-4) Lich summons three. I'm not sure which difficulty below arcade default reduces the zombie summons to two but that'll probably be the one that is closest to Apprentice. Unless it was just an arbitrary CoM change in the first place haha.