> How would they have done that?
There were a number of different, complementary means envisioned, the simplest of which was reflected in the chant “Hang Mike Pence”, which existed because the pro-coup political intelligentsia had proposed that the Vice President could and should simply refuse to open the certificates from selected states, denying the Democrats a majority of counted EVs, or alternatively could choose unilateral to accept “alternate” slates of electors from some of those states, an approach Mike Pence rejected. Removing Pence would result in a different presiding officer who might be more amenable to the plan.
And, sure, that still seems like a pretty dumb plan, but an insurrection doesn't stop being an coup attempt because the plan is stupid.
> In a coup, someone like a general is waiting to hear the signal to throw in with the coup leaders.
You’re thinking of a coup in favor of a faction that doesn't already hold the cheif executives office. An auto-coup, which is what was intended, usually involves some mechanism to provide a pretext to either extend power beyond it's lawful limits or extend it beyond it's legal term, not to seize it from the current holder. The only person you need to ready to do anything is the existing executive, who just needs to accept the offered pretext to retain power. Which Trump already announced his support for this theory and his anger at Pence for refusing it, which was a major contributor to the insurrection.