[rpeden]

They did a lot of static testing of both the SRBs, external tank, and main engines before the first flight. This NASA slide deck gives a pretty good overview:

https://www.nasa.gov/pdf/552577main_Shuttle_Propulsion_Mitig...

After all of that they were probably confident enough in the individual pieces working well individually that they were also decently confident that they'd all work well together during the STS-1. Although STS-1 through STS4 had ejection seats installed for both crew members, just in case. Perhaps the thinking was that if anything was going to go wrong, it would happen below 80k feet where the ejection seats would still be usable.

[dylan604]

"–Up to 20 tons of payload capacity on re-entry •1,693,500 lbs returned to earth*"

I always find myself laughing when propaganda material like this mixes units for the benefit of sounding more impressive. Capacity of 20 tons, yet they switch to pounds to get a larger number. 1,693,500lbs is only 846.75 tons, which just doesn't sound nearly as impressive.