Those are all valid points. Though I will argue that the assumption that the Capitol Police answers to the legislature is shaky, at best (in practice). If I had to guess who is more likely to refuse an unlawful order, I'd say a member of the military, rather than the police, based on my limited knowledge of their respective cultures. Combined with the idea of police officers who believe they are part of a semi-secret, ad-hoc, patriot's army, things can go wrong indeed.

Let's do a thought experiment: let's say there are a few elements in the police who are active QAnon believers, sprinkled in at various levels. Let's also assume some more force members are not believers, per se, but sympathize with the cause, and are willing to look aside since they may dislike some legislators who they see as enabling BLM, Antifa and other un-American actors (in their eyes) and believe that something "weird" happened with the elections and/or the whole establishment is dirty. Would these individuals not listen to the orders of the commander in chief, even when not delivered via the official chain of command?