I've managed to find a stable fix and make Reason 7 work on Sierra(no bugs and fully stable) and now Mojave as well(some visual bugs and stable so far), so I suppose the method will work on any OSX and possibly with any outdated version of Reason.

Steps:

1. I reinstalled the stock OSX (Mountain Lion in my case) that comes with the Macbook factory - Internet Recovery boot.(alt-hold while booting + wifi/ethernet + cmd+R)
2. Then I installed Reason 7 first thing and opened a project.
3. Updated OSX to latest version (in this case Mojave).
4. Open Reason. All is good.

Some notes:

Visual Bugs(probably because of the new black/white theme algo):

• the icons on the main open app splash for authorisation / online log in / demo mode do not seem to show up - everything clickable tho.
• the right-side overview(rack/mixer/sequencer) visual-scroll-bar appears to be blank but works fine.
• also the in-app browser doesn't seem to show the native osx folder icons. Other than that all is good.

Worth mentioning:

• In case your icloud/apple account doesn't seem to log in or sync after update (this seems to be a current issue on latest osx versions) you can fix this easily by creating a new admin account and just migrate (preferably at the beginning) to this new account and delete the old one.
• I recommend upgrading to the latest versions of course although some older macs will not work well with more recent Mac OS, also Reason compatibility or overall performance is tricky to maintain for some of us, so until then this is the best solution.

I've managed to find a stable fix and make Reason 7 work on Sierra(no bugs and fully stable) and now Mojave as well(some visual bugs and stable so far), so I suppose the method will work on any OSX and possibly with any outdated version of Reason.

Steps:

1. I reinstalled the stock OSX (Mountain Lion in my case) that comes with the Macbook factory - Internet Recovery boot.(alt-hold while booting + wifi/ethernet + cmd+R)
2. Then I installed Reason 7 first thing and opened a project.
3. Updated OSX to latest version (in this case Mojave).
4. Open Reason. All is good.

Some notes:

Visual Bugs(probably because of the new black/white theme algo):

• the icons on the main open app splash for authorisation / online log in / demo mode do not seem to show up - everything clickable tho.
• the right-side overview(rack/mixer/sequencer) visual-scroll-bar appears to be blank but works fine.
• also the in-app browser doesn't seem to show the native osx folder icons. Other than that all is good.

Worth mentioning:

• In case your icloud/apple account doesn't seem to log in or sync after update (this seems to be a current issue on latest osx versions) you can fix this easily by creating a new admin account and just migrate (preferably at the beginning) to this new account and delete the old one.
• I recommend upgrading to the latest versions of course although some older macs will not work well with more recent Mac OS, also Reason compatibility or overall performance is tricky to maintain for some of us, so until then this is the best solution.

Update: Found another visual bug - the pianokeys or drumkeys in the sequencer song-edit mode do not show up, again clickable but not visible. (This is a major nono for me so I had to revert to sierra which is best so far) Also I wouldn't call Mojave stable anymore - 1/10 times Reason fails to open in a crash, although opens right away the second time. Sierra is the way to go.