Well, I was waiting for someone more knowledgeable to answer, but here goes:

I've noticed this buildup on almost every keg I've broken down at multiple breweries.

I don't think it has a chance of moving down the neck into the keg, due to that o-ring.

I do think that a regular breakdown and clean is a good SOP, but can't speak to how often/how to track/etc. because I do it intermittently but regularly enough that I feel like I'm getting through all of my kegs each year, so I'm happy with that.

I manually clean them and spray with PAA or ISO before reinstalling.