Sorry for reviving this old thread. Just to let you know that I
finally made the cut with the jigsaw on my countertop!
I used all the suggestions you gave me to make a straight cut (good
Bosch blades, very light forward pressure, no orbital motion, a piece
of wood as a guide, masking tape on the cut line, some protection
under the shoe of the jigsaw and above all, PATIENCE!).
The final result, though not perfect, is quite good!
The only difficulty I found, was in keeping the blade vertical: after
some cutting, the blade, while going inside the wood exactly on the
cut line, had a tendency not to cut exactly up-down. Maybe it is the
thickness of the wood, or I was pushing to hard against the guide and
didn't realize it. Fortunately the beveled part of the countertop is
not visible.
Anyway, considering I never had a jigsaw in my hands before, I'm very
satisfied with the outcome!

Thanks again for your help!