I have not flown ex-YYZ to the US that recently but not that long ago and for non-connections, I think the OP checks in with United and goes to the US pre-clearance with her bags and drops them on a belt before security, not at the UA counter. The they can call you to the bag is the connection process. I never check bags but I do recall a belt after checkin, not at the counter.

Regardless, she will have to get her bag on arrival and find a place to sleep and at checkin with UA it will be obvious if they take the bag or they don't. The last ex-YYZ flight to MIA, AA was not even open so I got a BP from a kiosk. There was no bag drop at the kiosk so it has to be later than the checkin counter. And before security for obvious reasons.

There is a hotel across from the departures level which is the easiest though not likely to be the least expensive. The remainder are nearby and use shuttles which can be a major grind to and from.