I think this is good, although does not really tackle the original issue, in my opinion. An individual action is still using a d20 roll-under system, so there's still the problem of that being basic. Either way, though, here's my feedback on the Action Chain system:
Action Chains Themselves
In my opinion, there are a couple problems with what you've proposed. Firstly, if one failure warrants failure of the entire Action Chain, the chance of succeeding should NOT decrease. As a player keeps rolling, the likelihood of a failure increases regardless of any modifiers. Sure, not individually, but as a group, the more rolls you are making the higher likelihood one of them would be a failure.
Secondly, in my opinion, a multi-part challenge is not really demonstrated well with a roll-under system. As much as I love them, I do not think they give this kind of system justice. From what I've seen in systems with something similar to what you've proposed in-place, there is generally a way to tally successes (and get multiple successes in one roll) or are roll-over / roll high. In either of those cases, an Action Chain would have a total number of successes / sum of rolls, and as you make rolls, you contribute towards the problem. This would both solve the problem of being tricky with multiple players and having surprise obstacles occur. It also allows those that didn't roll as well to contribute, which seems more heroic and team-based to me, which seems like something you're going for.
My Recommendation
From everything you've described, if I were you, I would make the game somewhat Rooted in Trophy, but go in an opposite direction for Devil's Bargains. Trophy, in unfamiliar, is based on Cthulhu Dark and Blades in the Dark. Devil's Barigains allow the players to actively root for their own terrible end by giving a player a boost in their roll with a condition attached—they must do something negative or something must happen in order for them to get the boost. When I say go in an opposite direction, I mean that players, instead of pitching to another player something bad to happen, they strike a deal in order to facilitate teamwork. For example, if Superhero A was making a really important roll while Superhero B needed help lifting a rock over an innocent bystander, Superhero B could ask Superhero A if they could help them for a moment after their roll to lift the rock. With the permission of the GM, Superhero A can take an additional die as long as they help Superhero B.
Expanding on that slightly, for Action Chains, I'd use the Trophy resolution system, but take the result's number and mark it against the Action Chains' total. When certain thresholds are reached, the problems are solved.
Hopefully this gives you some food for thought! Very excited for what you turn out.