Had Pence been killed, or scared enough to change his position, then they would have been successful.

Okay. Let's game this one out.
We have a couple of dozen nutjobs heavily armed with pepper spray and signs. They're going to bust in on Pence, grab him and take him out.

By the time they got to the hallway where Ashli Babbit died, there were dudes in there with M4's standing right next to the mob. Then there's whatever armed detail Pence had with him. How would that have gone?

As it was, they did manage to prevent Joe Biden from being certified as the winner of the election for at least a couple of hours longer than would have happened.

Wow, that's some real End of The Republic stuff right there.

Had they managed to stretch that out from hours to a day or two (unlikely but not insanely so), it would have created enough space for something to happen that might have led Pence (or enough more Congresscritters) to jam up the process.

Uh, huh. Like what?
The democrats have objected to every. single. GOP. Presidential victory in the last 20 years. How much did that slow down the process?

But storming the U.S. Capitol with all of the elected members of the federal government (save the President) inside of it while doing an essential act is probably the best of all scenarios for interfering with the transition of power.

Really. You're aware that most buildings in DC are connected by tunnels so you can get VIPs in and out whenever and wherever, right? That takes the CAPITOL UNDER SEIGE BY ANGRY MOB WITH STICKS AND PEPPER SPRAY kind of off the table. I don't think there's a line in the Constitution that says Congress can't certify the Presidential election with a voice vote in the hallway of say, the Smithsonian or the Treasury building.