Just to make sure i understand.
1 your Jeep has oil in the Coolant/radiator , I’m assuming you opened the radiator cap and the oil had floated. They don’t inject Oil into the radiator coolant, I don’t mean to be rude, but that made me laugh, I wouldn’t tell my mechanic that, he’ll think less of you.
2 Your oil was a quart low?
Well, your Jeep, I would guess, is in for some Major work. Cracked head most likely (but I normally hear coolant in the oil). You had a coolant leak, so your cooling system was compromised. The engine was running hotter than it should, and problems happen from that.
If you have oil in your coolant, means there is a crack or serious malfunction and one system (oil) has connected incorrectly to the coolant system. That’s usually big dollars and work to fix.
The 1st problem, I would guess, caused the second. Running hot cause it, they fixed the radiator, but the next problem was caused. I wouldn’t think it was the mechanics fault. if you’re lucky it’s a gasket failure and not a crack.