Greetings BBB! Before I would start on the lever, I would start with the simple stuff first. You mention a sore thumb from trying load it. That could be one of two things or a combination of the both.
With the gun empty and the bolt closed, can you flex the Loading Spring (31) until it allows the necessary opening for your cartridge. In other words, is the restriction at the "gate"
1 or as the cartridge moves forward
2.
1The work on the spring itself is started by first checking to ensure that the attach screw (38) is tight. This is one screw that when loose creates a bit of havoc. If the spring is tight, it can be adjusted by carefully changing its rectangular shape to that of an hourglass with file and stone.
2When a cartridge will not enter the tube, different than the Loading Spring "gate" not allowing it's movement, remove the Magazine Plug (33) to see if the Magazine Tube Follower (20) will freely drop out. In the forum topics there are numbers of reports that have issue with the plastic follower. I don't think it is so much the material that it is injected from but the rough edges and restrictions that it is forced through at the factory. It is my belief that if the magazine's rough edges were chamfered correctly and the interior of the tube properly polished, a synthetic follow would be great. Once scared up though, it remains so and it's operation remains rough.
Some of tubes are reported with a restriction. The best way to correct this is to remove the tube, find a socket that has an outside diameter that matches the inside diameter of the tube, lube it up really good, and then force socket through the tube with a near full diameter wooden down. From there, I polish the interior of the tube with fine lapping compound and then very fine steel wool. Finally, I chunk the roughed up Rossi follower and ordered a stainless steel follower from
The Smith Shop. You might as well get a SS magazine spring from him as well. I like the stainless in the tube as the interior of the tube it is not well finished and you are constantly introducing contaminants with the cartridges. You will find less corrosion or deposits created by the trash with stainless steel. You did not mention which model of RG you had but the follower and spring will be $35 (includes shipping) for the RG4570 and $29 for the RG3030.
This is what I would do before I touched a lever. Oh, welcome to the forum!