I'll start with the easy one.
Issue #2:
ZEM changed on March 25, 20202 <--Click for patch notes. From 150 (200%) reduced to 112.5 (150%.) So your tests of 115-118 are about right.
Issue #1
Level 59-60 takes most exp. Level 54-55 is almost as bad as filling up 60 bar. Besides those two, 54-55 takes more experience than any other level. So I have no idea why 55 56 57 would be more exp than 54.
However, once the mob goes green, there might be an explanation: Titanium client exp calculations.
Green mobs that give exp are technically "light blue" by the modern EQ standard. Light blue mobs have an exp nerf. If you tracked which mobs you killed for exp that conned blue vs green, you could tell us what the exp nerf may be.
Intriguing.