It sounds to me like this was a bit of a logistic nightmare for the city.
Adding MUP signage is extremely cheap. A couple of cans of paint and a few sign posts, it probably would cost less to add signage to the whole corridor than it would cost to put in 10 feet of sidewalk.
Many municipalities require left turns to have designated turn lanes (there apparently was one in this case), otherwise left turns are not permitted.
Is there a center median strip where they could put signs?
What I'd probably do is repaint any "Zebra" crosswalks to be an alternating white/green. Designating it as both a bicycle and pedestrian crosswalk. Or, perhaps split like this.
Then add some signs similar to this in the median strip.
Of course, it doesn't mean all drivers will see and respect the signs, but it is a start.