RJF,
> On commercial macsyma, I get for the first problem,...
> I then ran the first problem on maxima 5.9.0 and it got the
> same answer.
Commercial Macsyma is getting the correct answer. Maxima 5.9.0 on my
machine (GCL, W2k) is not getting the same answer; it has DX where it
should have coeff_y, which is not correct. What is your 5.9.0
configuration?
I tested without the assumes, which as you say are not used by solve.
-s