Overreaching can be protected by bells etc. We had this one horse at training - he was overreaching and clashing his shoes together all the time when at work - he had all 4 leg guards and all 4 bells.
Now, he only clashes his feet when not asked to work properly, he is very heavy on the forehand, but active in the behind, so his hind legs are doing it right, but he hasn't figured out completely yet how to immediately lift off front, or the rider cannot ask for it. The trainer rides him without crazy noises. He does compete in dressage and jumping, low levels - in dressage no bells for the show - but no injuries either.
If the horse does it on his own too, always put leg guards and bells on for turnout, we do that for shod horses anyway..
For part 2)
Ad lib hay and water, salt/mineral block, and maybe a bit of high fibre and protein, but low on cereals and molasses mix - feed oil seed meals, sugarbeat pulp, can add some oil, make sure vit& min are enough.
Cereal grain only for fast release energy if needed.
To build muscle - lots of long&low lunge work and free movement. When he feels strong enough, ride long and low, work with transitions etc - muscles get build with more work, relaxed work, than just feeding ofc :P