The way I read it is that it’s distinctly different from Conan or Star Trek—that you can’t use it the way you described. If I want to use momentum on my attack I can repeat the momentum spend to get up to 3 extra dice. Now I’ve used the momentum this round so if I want more damage I can only get it from the doom pool. Then when I’m attacked later that round I want to defend myself, again I’ve already used momentum so it’s doom only. I asked last year after running the game and having trouble and got that confirmation.
I don’t want to house-rule it out, I want to understand it and use it correctly. It’s there for a reason. I don’t want to mess with it without understanding it and I feel like I’m missing something because it didn’t feel right. That’s why I wish there was a “working as intended” example of play that didn’t ignore the rule.
There’s a few other replies about it in that thread from last year.