Agreed. I'd also like to see Reisen as a "protag".
let's not forget that Youmu, like Sakuya and Sanae, are humans (or at least human-ish enough), so they fill the roles of incident solvers on the "main" games (or at least what was described as "imitators" of said incident solvers). Though Reimu quite did fill the role of incident causer on SSiB (involuntarily, or at worst, was just some "stage 4 boss" to the real incident causers.. in b4 "when will Chang'e ever make an appearance in a game, if she does at all?")..
point being, human protagonists fill the role of playables better (in ZUN's opinion) because their
PoFV PoV are more understandable than youkai's. I'd say that the spellcard rules kinda require an incident causer, and an incident solver, but Marisa's motivations/goals in UFO (or even Reimu's carefreeness in EoSD or UFO, but not limited to) may mean that it's not necessary for a playable character to actually want to fulfill the role of an incident solver.
@ Reisen in PoFV: iirc she was searching for Tewi, or doing some errand, not being focused in the game's incident (like, for example, Yuka, even if she wanted people to not think she was the one responsible for the incident)