OK, considering you said it started out hairline, like the other cracks, and there are multiple horizontal rings, I would suspect a nutritional problem is likely. Then mechanical stress/imbalance &/or infection getting in has exacerbated it. It appears it's not that bad yet. So what I'd do is...
1. Look at diet & nutrition & ensure your horse is getting a healthy, balanced diet. *Consider magnesium especially, especially in light of his 'soft soles'.
2. Open it up, as little as possible but as much as necessary, to clean out & treat any infection. It will probably require ongoing treatment. *Not seal it up for cosmetics with epoxy or anything else unless you're absolutely 1000% positive you've got rid of any infection and have sealed it fully against any further spores or bacteria.
3. Relieve the walls at the ground surface around the area of the crack and remedy any imbalance/pressure elsewhere which will cause more pressure/leverage on the wall. IMO this is much more difficult if shoes are used. Putting clips on or otherwise holding the very bottom of the crack together is contraindicative IME & will put the stress further up. Especially as it is the entire length of the wall, clipping or otherwise bracing across the face of the crack may be a good move to stabilise it, at least until it's grown out to about half way down.
As you've said the new farrier is, well, new, don't blame him for it. It's obvious the strain on the hoof walls, from whatever causes, has been going on long term without being addressed adequately & this is just the end result. So saying, he won't be able to 'fix' the problem by removing the flares, especially if you want to keep the horse shod.
If the horse has 'soft' soles, IMO there's another reason to get the shoes off, for now at least, until they can be improved(& I don't believe genetics is the problem), but rim shoes won't prevent abscesses any more than stone bruising, so it sounds like the horse definitely needs protection for the bottom of his feet, whether that's pads under shoes or boots, casts, whatever.