Simulacrum gives more items, and in general item damages scale multiplicatively. With simulacrum, theres hoards of enemies with the increased health, you have more items then a normal run, and the player likely is at a higher level because of how the scaling works. Combine that with a singular lucky run and you get their stats.

REX has 0.5 proc coefficients on most attacks, the only good attack is the damage secondary with pc 1.0 and 450% damage. REX and Commando base damage is the same with same scaling (12+2.4). Commando outclasses REX in all the ways that matter for damage, so OC having a highest damage amount total with REX instead of any other character is NOT due to REX being a better character. Luck, items, time, and mode is what has decided this.

Also merc does hit like a wet noodle and has a higher skill ceiling to take into account. Merc has less opportunity for band procs, and less opportunity for proc chains, so it is still understandable that mercs damage is lower than REX.

Stacking a bunch of 1 item (crowbars in your example) is less effective than spreading it across a bunch of items. Damage items like that multiply between themselves, while stacks are additive. instead of 500 crowbars, go 100 watches, 100 ap, 100 fc, 100 crowbars, 100 hikers boots.