Why in the name of the Mother, the Warrior and Jesus Christ himself would the faith choose a fucking limp gravedigger as their champion?

We don't know what the gravedigger gets up to in the time before Cersei has her trial. There could be any number of reasons for him becoming a candidate, but any reason would be somewhat contrived. But trying to find non-contrived ways to hit his story beats is probably why GRRMs taking so long.

It also undoes literally all of Sandor's character development and turns him into the exact same person he was when we first see him, which, unless GRRM really wants to go for Sandor as a tragic character obsessed with revenge on someone who isn't even alive anymore, would be a poor development.

I always thought his arc was about becoming his own man rather than just an obedient Hound. He always wanted to kill his brother, he just never did because it's not what his masters wanted him or ordered him to do.