They make some boots with gators that I've read stay on pretty good. I actually did a search for hoof boots here last night and there were a few people that used em consistently for trail riding through mud and water and rocks etc... And they noted which model they preferred that would stay on through all of it.
I have the regular easyboots and in the winter they would fall off. And I think it was from the rear hoof coming up and stepping on the back of the boot and causing it to pop off. But gators are said to stop that from happening.
And your horse will transition quicker with boots and pads. Don't forget the pads, they're the magical bit that makes it work so much quicker.
But it's hard to say how long it'll be before your horses feet toughen up. It could be a couple weeks or it could be a couple months or even a year. And it all depends on the current condition of your horses hooves and wether or not you use boots and pads. But with boots and pads you can ride on gravel from the first day typically, so you won't have any down time.
Another thing is movement. Lack of movement with proper heel first landings will cause it to take much longer. But lots of movement WITH heel first landings will cause the process to go so much quicker. It's this that will cause them to grow a stronger foot quick. But it's imperative that they are having heel first landings.
And with you saying that even with soft footing your horse seems sore causes me to wanna ask....thrush?
Another thing with your horse being sore in soft footing is possibly the heels contracted while in the shoes and now the frogs are small and withered?
A paddock paradise will allow your horse to get lots of movement which will also speed up the transition period. They'll just be out there all day walking around. This is much healthier for em compared to just standing in the field.
Proper trimming and proper movement, "heel first landings", are what will speed up the transition.
Do you have any professional barefoot trimmers in your area? They can take a look at the hoof and have a good idea what's going on. Here's a site with a, "trimmers list", link on the left side where you can see if there's someone in your area. The Horse's Hoof, News for Barefoot Hoofcare
But it's hard to answer your question without lots of details. Though I can confidently say that your horse will transition much quicker with boots and pads than without em.