Hi and welcome to the forum! I don’t have your background, but I think the transition might be fairly easy. I have actually had a violin background and occasionally played viola. What I found most challenging was the switch from treble to alto clef in music reading. I played in a student orchestra (as an adult) for one or two seasons while my children were involved in it. While the music was never really technically difficult, the reading never came naturally to me. The other things I can think of that might be worth considering is that the viola is heavier, so for you, moving to violin will probably feel easier in regards to holding with the head, shifting, vibrato, all the technique that requires you to hold with the head. Also, the intervals between fingers will seem a little smaller and of course that becomes even more pronounced in upper positions.
I have not had experience with 5 stringed acoustic violins, but the topic has come up in this forum. Those who have had experience mentioned that there was less standardization of size, related specifically to spaces between strings, and having that extra string perhaps the maker tried to squeeze all strings into the same space as four strings. Envisioning that, I think it would be a nightmare! I would rather the maker just accommodate the additional string with a wider fingerboard, allowing for strings to be spaced as they are on standard instruments. Brings new meaning to the term ‘try before you buy’ !
As far as teaching students starting on viola, for smaller students, I would suggest re-stringing violins or playing on violins until they get used to holding with the head. Then introduce the heavier violas.