arugula: Rest assured that training will not affect disease or insects unless you block light or air. Cold isn't an issue either. You can train now but the branches won't "take" in their new position until they grow new tissues in the shape you set. When training limbs down I tie off to a stake or weight resting on the ground. Hanging weights aren't stable and can break a limb. By fall the limbs will be set in their new position.

Your descrition made me think of my 2nd leaf Goldrush on G11/MM111 interstem rootstock. I probably did too much branch bending on this one last year. Since it was already bent close to horizontal when the fruit formed, it didn't have anywhere to go but lower... An interesting question is where do you measure the trunk diameter? Mine is just over 1" at the ground, increases to over 1.25" at the interstem, and is only .75" above it. I checked online and there seem to be a variety of answers (30cm, breast height, 1.5M, etc). 1.25" diameter equates to 7.9 cm^2 and 31-47 apples, obviously too many for this tree. .75" diameter equates to 2.8 cm^2 and 11-17 apples. I'm guessing that this is the better value to use here. I thinned it pretty well, but only 1-2 apples near the end of a long branch can really bend it down. My tree currently has 15 apples (10 bagged), so it is right in that range. Soon enough, I may need to tie it up, instead of down.

You likely overpruned this tree, stimulating too much shoot growth. When this happens, there is a lot of new fruiting wood, and a lot of fruit is produced, often smaller and more numerous. This leads to broken and bent branches. You can reduce the number of these new sprouts, and reduce the length of the remaining ones. This will reduce the number of apples produced, and will make the remaining branches stronger. Also, by thinning the apples in their early growth, you can make larger apples. Pull one apple off of clumps of two or more apples. Good luck!

Sorry about the damage to your son's tree. Not fun. I have to say, I am not a big fan of 'Desert Museum' palo verde trees. I know they are the "in" and trendy tree right now and are being planted everywhere, in commercial settings especially, but I think they are awfully lanky and gangly and subject to a lot of wind damage. They're popular because they grow very quickly, and low litter and generally thornless, but that quick growth makes their branches shoot up and look boring and weak (to me). I know, I know, a ton of people have them and love them, and I think the best ones are ones that get regular pruning and not too much water. I'm not saying I'd never try one if I ever moved to a new garden, but I would expect to have to prune it a lot to keep it looking nice and to keep it from getting storm damage and honestly, I just don't think a lot of plant sellers are telling people that part. Your son can prune his to try to create a shape he likes and that's a decent approach. I've seen some very nice ones around town, but again, they're getting regular pruning to thin them out and to make interesting branches. If he replaces it, I'd suggest getting a non-hybrid native Palo Verde if possible. In any case, he can defitely give it a good haircut and see how it progresses. Thinning out and removing a lot of the thin branches will help its appearance for sure. Good luck to him. Let's see what our other members say too. Take care, Grant