Multipliers from the same source stack additively, while multipliers from different sources stack multiplicatively.
The 600k score is more than a week old at this point. At the time, there was no alchemy and only one hero available. Cheesing for 9 screaming mandrakes seems absurd to me. That's like (0.06 x 0.05)^9 (probably a little less rare since the correct calculation would use a binomial distribution?) Even 3 screaming mandrakes is already super rare, having all those 3 trigger +9999 vitality is just so astronomically unlikely... unless I made a mistake and accidentally set the chance to something like 95%. (Yeah, there will be a recruitable hero in the future.)