its been a few months since I practiced it. I forgot the input 
but seriously, holding down does help against the smaller characters like mag\storm. I think landing the infinite with doom depends on your team.
some characters like strider can easily setup dooms infinite. If I hit someone with orbs and they are no more than 25% from the corner, I can use strider’s orb time to dash them into the corner, tag into doom, infinite, photons, dhc orbs. I’ve used this in the past when I ran S\D pretty hardcore and have pulled it off quite a few times. Really useful for S\D because the infinite builds some decent bar.
IM\C\D can pretty much setup it too. IM infinite to the corner, PC, dhc HVB, pop them up a little so dooms dhc photons super hit on the top row. Dash, launch, infinite, fly photons, DHC back to PC.
I’ve pulled off the doom infinite VS alex garvin in tourney play about a year ago. He’s not really known for marvel but he’s pretty good @ it. He’s a well known GG player though.