After many years without LazyMouse, I was inspired by your Post to try again. I can happily say that I got LazyMouse 2.5 running successfully in MacOS Sierra 10.12.3.
I downloaded Version 2.5 at this link: http://mac.brothersoft.com/lazymouse-download.html
I completed the Install process. It installs as a System Preference, not an app. When I tried to open it, I got this:
This is referring to an older version of Mac OS X. Instead, go to System Preferences, Privacy, Accessibility and look for Lazy Mouse on the right side. In my case, it wasn't there so I did a Restart.
Even after the Restart, Lazy Mouse still did not appear in the Accessibility window. Attempts to open the Lazy Mouse Preference Pane resulted in the "LazyMouse cannot run if "Access for Assistive Devices" is turned off in the Universal Access Preference Pane" as seen above.
I more or less gave up and walked away for long enough for my iMac to go into Sleep Mode. Three hours later, I went to System Preference and double-clicked the Lazy Mouse icon just to try again. To my surprise, Lazy Mouse appeared to be working as I got an active window. Since I had previously purchased Lazy Mouse, it even showed my name next to the "Registered To:"
I checked the "Whenever a new window appears, snap the cursor to the "Default Button." I then went back to the Privacy, Accessibility window and again, to my surprise, Lazy Mouse was now there as seen below:
I "checked" it and did a Restart. After the Restart, I went back to the Lazy Mouse Preference Pane and did the "Try it Out..." That worked! I then opened a document in Pages and went to File, Print and pleasantly saw the cursor on the blue "Print" button thanks to Lazy Mouse. I next tested it in Preview and it worked there as well.
I hope this info helps someone attempting to get this "classic" Mac app up and running in Sierra.