Let’s take the second question first. If the Friesian’s Rein Back 1) comes from a balanced, immobile, and square halt; 2) if he backs maintaining a round topline without resistance; 3) if he marches back in unconstrained, relaxed (appearing to be) diagonal pairs; 4) if he stays straight; and 5) if he takes the desired number of steps and moves off promptly without squaring up first, you have just made a really good Rein Back! If he steps higher as a function of his breed characteristic and not from tension, the judge should totally ignore it.
As for the first horse. Rein Back is a wonderful measure of suppleness and thoroughness. A horse that hurries back, that runs back, or that drags back is saying he’s not convincingly on the aids. You’ve undoubtedly heard that Rein Back is really “riding forward in reverse.” It’s distinctly different from just backing up. It can be especially difficult to persuade a hot horse to relax and to wait for the aids so he’ll step back rhythmically, promptly, and deliberately. In the beginning maintaining his attention and pliability should be at the top of your priorities. An instant after you apply the leg, a momentarily non-allowing hand invites the horse to step back. As soon as he begins his step, your aid must soften to reward and then repeat for the next step. Be clever—pushing with both legs in unison or resisting symmetrically with both hands at once often isn’t the most persuasive approach. Channel your horse back, keeping him straight by eschewing generic aids and using a stronger or softer leg or hand as he reveals his needs. Be sure each step is individual—that after any given one you can stop, or go forward again, or pause and continue back. Horses can get good at “counting,” I once bought an FEI horse who could make a decent looking RB as long as it was exactly four steps. Nothing more, nothing less! It appeared fine, but it was completely fraudulent and of no gymnastic value. It was only a trick! It took quite a while to deprogram him to make a proper one.
As a side note, the 2015 USEF tests at Fourth Level Three have re-introduced the schaukel or “swing.” It’s a back four/forward four/back four movement to be done fluidly and with willing acceptance that’s a really good measure of your horse’s thoroughness at that level.