Thanks, Davide, for the helpful comments.
I have attached my current version for your second example.
a) I opted for your third alternative --- it does not attempt to replace 12/-5 with -12/5 nor -12/-14 with 6/7. I may revisit this after your revision to Fraction objects is in place.
Having a student answer display as decimal poses no difficulty, especially since Preview will show a typeset version.
c) I opted for your second version, primarily to avoid having to create a diagnostic error message for a student who misread 2y-3x as 2x-3y.
b) OOPS --- my intent was to say that I am content with the current status: no comment about possible dissonance between an acceptable student response and "correct answer(s)" displayed by the ShowAnswer checkbox.
d) I will post an Extended GCD function in a separate message.