Btw, regarding the mid boss I just saw this on discord: "Rejuvinator buff gives 50% less respawn time on your next death, makes creeps 50% stronger, increases fire rate."
I believe it's random. If duo queueing there's some priority to go into a double lane, at least that's what I've noticed, but besides that I haven't noticed any pattern.
Just tested this in sandbox and it doesn't expire. It's permanent throught the entire game even if you die.