I can't post pictures because they aren't my pictures, but a good riding buddy of mine just went through a similar battle. Although, her white line issue is being cleared up pretty quickly by an amazing farrier, so she is lucky there. Regardless of that fact, she had no warning signs before it was full-blown, aside from a minor case of thrush that she had treated and cleared up. Essentially, a crack provides the opportunity for the infection to go deeper and once sealed inside there it eats away at the inside of the hoof in secret. Or at least that's how I understand it. Her farrier cut out the entire infected area (a huge portion of the hoof was missing after--looked crazy but the horse is completely sound). He then packed it with a blue hoof bondo type material, and put a custom made bar shoe on it. After doing all that the horse is completely sound to ride as much as normal...just some extra $$ from the owner to upkeep the healing process with special shoes and such. He said that with the amount he cut out it will take 6 months for regrowth, which means the horse will have to remain shoed for that time.
I am in no way shape or form an expert on this...just thought I would share a similar experience and what the results of it were. My riding pal felt plenty guilty as well and was also quite blindsided. Just make sure you have proper farrier assistance (which it sounds like you already dealt with) and you should be okay. It takes a lot of courage to share problems like this, especially when people are quick to blame....so kudos to you for seeking answers for your horse's sake even when it means biting the bullet yourself.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.