I personally would say 'no' on both counts. Lemmings aren't exactly elastic, and there are special interactions if the lemming has a permanent skill (i.e. if a Climber jumps onto the wall, they will climb).
Also, if Shimmiers were to turn around upon hitting a wall, how much wall should there be? For example, if there's a stalactite that's just 2 pixels tall, they can't shimmy over that, but turning around would be very strange.
What happens if the lemming is a Slider? Do they slide down the wall on both counts?
A few levels in Lemmings Cranium also depend on a Shimmier falling when they hit a wall, so I'm a little biased
I feel that changing this particular behaviour is an extremely fundamental change that will break a number of levels, especially with the Shimmier.
In Lemmings Cranium, Cunning 12, Cunning 19, Chaos 4 and Chaos 30 will break if the Shimmier turns around upon hitting a wall. In the case of Chaos 30 it will just be a zombie that will move back and forth along the ceiling, but the other levels depend on the current behaviour to be solvable. Granted, Cunning 15 depended on Gliders hoisting themselves up if they barely miss the ledge, but thankfully a red herring made this level still possible to solve with the new Glider behaviour.