I personally think that the only "limits" by the spell card rules are to make those abilities translates into danmaku play and the absolute rule of nonlethal-ness. Another factor is the characters' "willingness" to use their powers.
For Reimu, she's still intangible during Fantasy Heaven, which is what it's supposed to do and the only limits are the time limit, which applies to all cards. Not necessarily unique to that card. So you can't hardly say that the rule will nerf any particular ability, no matter how OP you think it is, because it simply was never the case.
So Yorihime can summon one or as many gods as she wants. Nothing prevents her to do so as long as it follows the rule of non-lethal attacks and the usual time limit, and seriously, why should it need to? If you say that the spell card will be impossible with multiple gods, well, why would it? We're already shown in ISC that impossible spell cards still counts into danmaku play(and still doable). So if there's any limit to the use of their powers in duels, it's the users' willingness themselves.
I don't really understand the fuss over Yorihime's ability when Byakuren can do the same with Buddhist & Hindu deities in HM and ISC, Kanako and Suwako are full-blown gods, and other characters with more powerful abilities compared to most gods never had to be treated specially in duels.
On Yorihime, though I'd like to think that she can summon multiple gods, it's never shown clearly so there're some doubts.
For the Sword of Gion, I think it's more like a god-infused artifact, so she doesn't need to maintain it like she have to in other summons. Which means it doesn't interfere with her invoking other gods.
