Ok, after the test above to run opera locally I am pretty sure that libgtk319 is not needed to run opera, it is just a packaging requirement. So for those who want to install opera69 in Ubuntu 16.04 you can just edit the debian control file to fix the libgtk3 requirement and then rebuild the deb. Here are the steps:
create a directory for editing and rebuilding the opera deb
Download opera-stable_69.0.3686.57_amd64.deb and put it in the opera_build directory created above
cd opera_build mkdir tmp dpkg-deb -x opera-stable_69.0.3686.57_amd64.deb tmp dpkg-deb --control opera-stable_69.0.3686.57_amd64.deb tmp/DEBIAN
This extracts the debian metadata in opera_build/tmp/DEBIAN, in it there is a control file
now edit the control file (still in opera_build)
change the requirement libgtk-3-0(>=3.19.12) to libgtk-3-0(>=3,18.9), then save.
Now rebuild the deb (still in the opera_build directory)
dpkg -b tmp opera-stable_69.0.3686.57_fixed_amd64.deb
this creates a new deb called opera-stable_69.0.3686.57_fixed_amd64.deb
now you can install this like you would normally with a deb and it no longer complains about libgtk3 version. Now when the pop up window comes up asking you if you want to update opera with rest of system, uncheck it, since next time there is an update you would need to go through this whole exercise again, apt upgrade will fail.