In case the Apollo Lunar module had approached the landing site at way too high velocity, could the ascent stage have separated and returned to the command module before a crash? Thus performing a task similar to a launch abort tower. One challenge would be to synch with the orbit of the command module.
Yes. There is a lot of good information in this presentation from the June 1966 Apollo Lunar Landing Mission Symposium relative to landing flight design including abort planning. The crucial figure is this one:
which shows the capability of the ascent engine to abort all the way down to landing. It assumes a 4 second delay to separate the landing stage.
As far as returning to the orbit of the command module, the Apollo Experience Report - Abort Planning document explains on page 24 (33 of the pdf) that the Lunar Module would attempt to attain a safe orbit and the Command Module would rendezvous with it.