Can an airliner land safely using only propulsion control?
Some airliners have in fact landed more or less successfully using only engine power for control. The 2003 DHL attempted shootdown incident is an example.

A major difference between United 232 and Fullerton's landings you cite is that Fullerton was working for NASA testing a system designed specifically for the purpose of assisting pilots in controlling an aircraft with power alone. United 232 had no such system. See https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-041-DFRC.html