Yeah, I was doing some napkin math trying to figure out how likely it is to get the 5 round run properly and gave up lol. Nice job on the 6 round run!
Some probabilities:
Round 1 damage:
5 damage: 2.5% chance
4 damage: 36.4% chance
Round 2 would be similar, but slightly better because you have stuff from round 1.
Getting 4 or more summon spells in rounds 3-5 is ~3.7% chance
Getting 7 or 8 or 9 rats/skeletons/summon spells in rounds 3-5 is ~7.5% chance.
These aren't all independent probabilities and I wasn't too thorough checking them, but you're looking at a 1 in 5000 chance to get what you need and even after that you need to play well to actually win. This does seem in the realm of doable legitimately though.
For the record, I haven't gotten a 5 round run without twiddling probabilities, so the challenge of being the first legit 5 round run is still open ;)
The probability of getting the market selections you need for the 4 round run is 1 in 40343846 alone. It's probably just as rare to get the enemy spawns you need so that's never happening.