I do agree with the others that, although it sounds like you were taken for a ride here (no pun intended), it is up to YOU to do right by this horse. I’m a firm believer that if you own a horse when the music stops, the only ethical thing to do is take care of him and retire him if necessary.
And I do agree that you should have brought the horse home the first time he failed a prepurchase, instead of continuing to market him and ultimately leasing him out. But what is done is done, and hopefully you will do the right thing now that you’ve become more educated on this topic.
By the way, injecting for navicular absolutely isn’t the same as injecting things like hocks. Often, you can inject hocks to help a horse through the fusing process, which ultimately results in long term soundness for the horse. With injections for something like navicular, you are only masking the pain which simply guarantees that the horse will further degenerate and injure the area.
A decision to keep injecting and working this horse, particularly in a jumping discipline, is, simply put, a decision to use him up completely and leave him a painful cripple. I won’t fault you for not knowing that from the outset, but you know it now, and what you decide to do with him and for him is a big decision that will say a lot about your character. Please choose wisely, for his sake and for yours. I can’t believe anyone wants to live with using a horse up in this way.