An end gap over ~0.018" to 0.020" might be viewed as end of ring life on a small engine. There's really no set number for these things but once they wear enough they just don't make a good seal.
I use an old broken piston ring to clean the ring grooves. Make a 45* bevel on one of the broken ends and carefully drag it around the circumference of the groove after dripping some cutting oil in the groove. I haven't tried it but some spray type carb cleaner might help out. I'm not keen on using a blade where the angle can be changed a lot because pistons are pretty soft, and enlarging the ring groove is not a good thing to do. I'm against using a screw driver for cleaning anything. An alternate would be to modify an Exacto blade by rounding the end to prevent a point from scoring piston at the back of the groove. Go slow, use care, there's no reason to race through the process. Spotless isn't necessary. In any event, you're better off not cleaning the ring groove at all if you suspect you might end up gouging on the metal.
The bottom of a ring pretty much never needs to be cleaned, the crud is always on top and on the inside edge. Given a choice between cleaning the inside edge of the ring and the inside of the ring groove, do the edge and omit the ring groove. You can scrape, carefully, the inside of the ring with a sharp blade, but vastly better is to use some semi worn out green Scotchbrite and some oil to remove the carbon. When doing the top of the ring lay it flat on a hard surface to support the ring. Don't use the Scotchbrite to hold the ring while scrubbing top and bottom at the same time. You do not want to round the corners of the ring, which happens if you wrap the cleaning pad around the ring.
If you've done it a lot you can cheat and do things really quick, but only if you've practiced a bit. 1,200 to 2,000 grit wet or dry polishing paper (auto paint store) laid on a very flat surface like glass. Apply a few drops of cutting oil to the paper, lay the dirty side of the ring in the oil, apply light and equal pressure to the ring and move it in an orbital manner on the paper just enough to clean the carbon, not polish the metal. Polishing the metal means you just made it thinner. 3 in 1 oil, ATF, Marvel, or similar can be used if you don't have cutting oil. Heck, even your 2 stroke mix is better than nothing.