Rebooking reward seat - will it appear back in the inventory

I made a rookie error and booked a return using my Credit Card voucher and miles, rather than booking it as two one ways. (I assumed with the voucher I could only book it as a return)

I now need to change the date of the return. But of course the outbound was booked on a saver fare in UC. If I cancel the original booking would I be correct in assuming the chances of that saver fare coming back into the inventory are pretty much nil and I now have to just suck up the fact that I'm going to have to pay more miles?

Correct.
Unless in a few days / weeks the points come down due to the dynamic pricing. when you cancel it will not release the saver seats price you originally bought at.

FYI. I just changed my flight today MAN-MCO from 16 Jan to 15 Jan. I checked before and UC on 16 Jan was selling at 41k points per seat in UC
I booked at 29k points UC, after I changed, I checked the reward seat checker and in the app and the seats went back down from 41k to 29k.