2020-11-10 : Because Stumpsquall Hydra's ability doesn't target, you choose how to distribute X +1/+1 counters as its ability resolves. Players can't take actions between the time you decide this distribution and the time counters are put on the appropriate permanents.

2020-11-10 : If Stumpsquall Hydra leaves the battlefield before its triggered ability resolves, you can still distribute the counters among any number of commanders, including none of them.

2020-11-10 : You can't put +1/+1 counters on commanders that aren't on the battlefield.

2020-11-10 : You may put +1/+1 counters on commanders that aren't currently creatures (such as Tevesh Szat, Doom of Fools).