Cape guy was most likely the super necromancer from the MC's previous timeline. MC is currently essentially stealing his role/glory and blocking his progress by being a better necromancer.
Regarding the 2nd last page: No, his ability has no limits. The only reasons why something could threaten him are his simp powered stupidity and incredibly powerful foes that somehow negate the time stop.
"Hey guys! Some psycho tried to murder me just now for no real reason in the broad...
Think of the classifications like this:
You put an item in a box ->
- If it will stay in the box = Safe
- If you don't know whether it stays in the box or not = Euclid
- If it tries to get out or it can't be contained (usually also hostile/lethal) = Keter
- If the item is the box = Thaumiel