Hi.
My advice about the "M" is that you let as it is.
The wall checking says here's an issue cause the software don't like pointed details and marks it, but in the manufacturing process this pointed edge usually gets a naturally softly.
Automatic fixing (blue) can't help you at all. Best you do is don't use it.
I think you have to use a wider font for the text. It's just in 0.4 mm., but you can design it in 0.45 mm and the results go over the limits and don't go to change the look at all.
But this model go be rejected as it is because the diameter of the sticks where begins the balls is 0.52 mm and had to be over 1.0 mm. Increase it to 1.2 mm, over the minimum unsupported wire limitation, and you go to be more sure.
Don't be worried if the auto thickness tells you some edge issues. The model gonna be manually checked before printed, and here is where they could find if there's real issues.
Good luck with your amazing design.
Best regards
Luis