The uninstaller works fine but the most common cause for installation errors is registry corruption and if the keys appear the same they are not over written. Blame windows for that one.
As to the extra content, you found, it's irrelevant to the operation of Sonar and is over written during a new installation.
The keys I gave you are the typical problem areas. I wasn't sure how clued up you were with registry hacks so didn't want to overload you with more than necessary. But yes getting rid of every trace when you only have 1 Sonar install is a good idea. For many of us we have multiple version coexisting. Eg I have 8, 8.5, X1, X2 and now X3.
Don't give up on Sonar. Despite the problems you're having by and large most issues are down to a conflict of Sonar and specific hardware/software interactions. Believe me I've been troubleshooting Cakewalk and other software for decades.
All the best.