How close are the reproductions to the originals? I would probably have to do both buckets (uppers and lowers) to make them match. (Which is what I wanted to avoid.) The original interior was really nice given the age. It was very upsetting when it split. I assume the vinyl got dried out. Not sure if a conditioner would have helped.
Reproduction quality varies. There are a number of threads that compare or at least have comments and experience with one brand (makers not sellers) our another
Personally if you have a really nice original interior I would try and keep it. Once you start replacing one item the differences will start to be very noticeable. Now if your doing a complete regular concours restoration where reproduction items are allowed, then original material often looks out of place and will often get some attention due to age and wear related details.
If you do one seat, yes top and bottom then the passenger seat will likely look different since its made of different material, thickness and so on. Then it may be door panels, carpet and on and on
Using the upholstery from a coupe back seat for the repair is an interesting idea. Although trying to find one still presents a challenge.
Just depends. Found a 69 rear coupe seat at the first place I checked with. Places are more than happy to sell things that are very slow movers