First of all, thanks for your fast response, I didn't expect an answer today.
The first problem is now solved (not sure how has been solved). I have changed the spline a bit, and also the last vertices are now bezier corner instead of bezier. The base object is now an editable poly, converted from line+extrude+UVW mod. Sorry, I can't reproduce the issue with the original base object (a saved copy), so must be the spline, but I don't the previous version. If you still want the scene I'll send you. Anyway, if the issue comes back, I'll save the scene in order to send you.
Thanks for your help and sorry for the false alarm. I'll apply your solution to the second problem.