...is it necessary to put the complete path in the variable [executable] (e.g /opt/frostwire/frostwire_noarchxxx/runfrostwire.sh ??)...
Absolutely, this makes all the difference. I believe the symlink would have worked if the full path was known and used. We were creating a symlink for a script that wasn't even there - no wonder it did not work.
as root: ln -s /opt/frostwire/frostwire-4.17.0.noarch/runfrostwire.sh /usr/bin/frostwire
But you need to make sure the bolded part is 100% correct. If the system complains that /usr/bin/frostwire already exists then remove that file first (as root, rm /usr/bin/frostwire), and then create the symlink.
edit: updated path in symlink based on your post below