Sounds like you are in a bit of a bind, sorry. Stretching hats I'm learning is an interesting business. Some will resize and reshape quickly and flawlessly and others just won't give in. For what its worth here are my suggestions.
Make sure and soften the leather with a good premium creme so it is more pliable.
Sometimes you need to overstretch a teeny bit as some leather is tougher than others and will want to return to its original form.
With the stretcher in the hat, give it a good dose of steam. The felt wants to hold the original block shape and is exerting forces as well. Especially in a subtle regular to long oval conversion.
Be patient. Leather is sometimes like a hatter, it gets set in its ways and is hard to shift. Keep the pressure on for a few months with a dedicated stretcher.
Don't worry too much about it ending up a bit loose. There are ways to shrink things a bit as well.
Send it to a professional if you are at all unsure. Although in your case, I'd try it yourself first.
(Suspecting his hostage hat is being tossed into the 6 month stack )