The issue of player-count when

[Brista]

Her Rune Drop is used when she does her No Drop, so it actually lowers your chances of getting runes if you kill her in a multiplayer environment.

I think this is not quite correct

What I believe happens is the game first checks her regular drop and then her special rune drop for each pick. If unsuccessful with both it then assigns a no drop for that pick

With more players each pick is more likely to succeed in getting a regular drop so it reduces the rune drop chance simply because for that pick it didn't get so far as to check the rune drop