Hi, Thomas. For the A chord, I have always used only 2 fingers, index & middle. It leaves more fingers for doing fancy stuff later on, too. :) I've even seen somebody play open A with just one finger.
For the other chord, I find that method of diagram hard to read, but it looks like an F. In trying to reconstruct your problem, I thought maybe you might be trying to keep your index finger more straight up and down than you need to.
For me to be comfortable on that chord, my index finger sort of has the tip up at the fret and the middle knuckle more back at the nut. If I try to hold the tip vertical, I start getting your problem a bit.
I don't know about you, but my joints are only so articulate. :rolleyes:
Anyway, maybe that will help to leave your index slant to a more comfortable position. ...And try to come straight down on the strings, keep those knuckles up!
...so ever since then, I always hang on to the buckle.