Nah, just go ahead and go for it, have fun and make those signposts feel real. Be careful not to limit yourself -- you should feel free to let loose here and use whatever your imagination comes up with. Don't worry about bleeding into OS or RS, that happens to me all the time and it just makes it easier to weave the throughlines together later! Obviously don't try to bring in other throughlines, keep it focused on the MC's personal issue(s) as much you can, but in a loose sort of guiding way, rather than feeling handcuffed. (Oh, and it's totally cool to have other characters "in" the MC throughline. There are a lot of stories where certain friends or family members are used for the MC throughline perspective. They can be OS characters too, or just part of MC.)
At this stage it's okay to make mistakes, you can go back and fix things easily. Once you get to Signpost 4 you may review the whole thing and realize the arc (another word for PRCO) will be stronger if you make changes to earlier ones.
Greg's right that the process needs to be the problem, but FWIW I definitely felt like you captured that. (MC has an idea, is trying to visualize how to implement it, and it's that process of visualization that's specifically causing problems because he's visualizing two mutually exclusive methods. I didn't think you were going for a choice between them, but more like the MC is feeling like he needs both to be happy.)
This is going to sound like crazy advice (and maybe it only works for me)... But personally I think Dramatica works WAY better when you're more accepting/inclusive in terms of what "fits" or "counts" for a particular element. I'm not sure if anyone will agree with this. But I feel like, if you're working from a storyform, and you have a good sense of your narrative & 4 throughlines, and an illustration inspires you and feels right, it probably is. And if something doesn't work, I feel like your mind will self-correct and let you know (usually via your gut).
Plus, if it turns out you had the wrong storyform to begin with and you're actually crafting a different argument, then the looser you allowed yourself to be with the stuff you wrote, the more likely you won't need to change it!