NORM - Yes, the only wear on the rectangular hole will be in those two corners. For whatever reason the end plugs that center the two springs push that way and the end plugs wear those grooves or slots. I have seen the other two corners wear but not as much as those two.
Yes, you can just try to weld up those two areas but they are so very close to the edge of that lever there's not much metal to absorb and dissipate all the heat of welding. Very likely that whole thin side of the rectangle will melt off where those grooves are wore, especially the one that's wore the most.
I've actually had the best luck using either 3/32" AWS 6013 or 5/64" 6013 rod on thin stuff like that. The 1/16 inch rod I mentioned has so very little metal in the rod you end up having to hold the arc so long you end up with more heat in the part. The 3/32" 6013 should be easy to find, I suspect the 5/64" just about IMPOSSIBLE.
You could use a small diameter, 5/64" or 3/32" 7014 rod but it's even less popular than the 6013. And there's also 7024 rod and to be honest, I've wanted to try it for 25 years and have NEVER been able to find it when I've looked for it! There is 6011 but it's for deeper penetration and you don't want that. There's also 7018 but you have to run it at higher amperage which is more heat and you don;t want that either.
All those rods I mention work fine on an AC or DC output welder. If you have DC output Your lucky, and you should use electrode negative or "straight polarity" when you weld this.
I'd also suggest backing the weld area up with a piece of copper to support the metal and dissipate the heat to FREEZE the metal quickly.
And both during and after the welding is done use a sopping wet rag wrapped around the lever to cool the lever so you don't melt the rubber lip on the shaft seal.