Lou,
There are a few questions here.
Firstly you can buy GAHH interiors by item, i.e. seat backs or seat bases without having to get the whole lot of the interior so they will be fine when you then go to do the rest of the interior. Be advised though, that ordering the GAHH gear through Buds Benz you can get a better price than direct from GAHH as Buds buy bulk from GAHH and provide a discount to our members.
However, without being critical of your upholster, a good upholsterer could unstich the piping seam and make a new piece of piping and sew it back into the seam. This would be done by removing the seat from the car and cover from the seat. Whilst you are there you can look and see if the springs have failed or just sagged. One way to bolster up the seat if it is a sag in the short to mid term is to put a “noodle” into the seat. Do a search of the forum for 'pool noodles' and it will explain how it is done. If you just fix the piping and at the same time whilst the seat is out, add the noodle, it may well be a very cheap fix that will last until you have the dollars to do the whole lot at a later date.
Garry