In the Army Of One event you can test this easily enough.
If you down 1 of the opponents heroes and leave 1 + Deadpool.
Just get the 1 hero to 40% (or whatever % is needed) or below and use Retribution.
As it’s supposed to, Deadpool will willingly stupidly jump in the way and down himself BUT, the order will be:
Downed - Not Downed - Downed
At which point I was going to change the order, but I decided to see if there was a bug here other than the hero order one I’m looking at.
So I used Chemical Reaction from Daken (as you know, this should do damage and remove strike tiles + damage for them), the damage is made on the downed hero which it then switches to the second hero that isn’t downed who is then untouched.
I will assume this will work with any ability, not just Daken.