Okay, went through my addon and replaced all my SetAttribute and GetAttributes to use spell1 instead of spell and added RegisterForClicks and the buttons don't work.
Aha, the RegisterForClicks appears to be the problem. If I remove that line everything works fine.
Removed the line from your main.lua file of Portable and your spell buttons work on left click.
Doesn't solve it full for your addon which relies on the right click option but one step closer.
Also tried with "AnyDown" incase the two button option was an issue but that didn't help either.
Aha .. seems like this might be the problem...
https://github.com/Stanzilla/WoWUIBugs/issues/268
Just testing that theory out
It worked for my addon but not yours .. back to square one .. *very puzzled*
Although based on report
https://github.com/Stanzilla/WoWUIBugs/issues/282 this was fixed several patches back..