Yes, they can shed seasonally. They can also shed quite a bit when your rehabbing a hoof, changing the weight bearing and the work load they are doing. They will also shed if the heels have gotten too high so there is no ground contact and wear on them.
Every frog is individual, but again, it has a lot to do with health, wear, remodeling, diet. If those feet were in shoes for a long time, the frog is likely underdeveloped along with the digital cushion, lateral cartilages and every other structure in the back half of the foot. Healthy frogs should have a rubbery texture that is firm, not soft - think like a super ball, not silly putty.
Make sure the diet is supplying all the needed nutrients to support healthy growth; trim is allowing for heel first landings; feet are in boots+pads so that landing correctly is comfortable otherwise the horse will do everything it can to NOT land on those weak, painful heels.
Time will be the biggest thing because you can't develop a strong, resilient back half of a foot overnight - it takes lots and lots of correct use to build it up into a good, strong, functional structure.