Probably the easiest and cheapest way for you to take those architectural shots is to get a tripod with rollers on it that you can lock so it runs straight.
Use a prime lens and push the trolley top the left side, take a picture, then shift the whole thing to the right so you still get overlap, then move right again etc and when you've got everything in the set that you want, stitch it together in Photoshop, or in Elements.
That way, you won't get problems with curvature etc.
A tilt-shift lens won't fix the problems you are encountering.
They are mainly designed to get a certain part of the picture in or out of focus, and for parralex errors, and won't neccesairly be able to get a whole room in perfect focus, without a lot of fiddling experimentation.
It's easy to find the nodal point of your lens.
Place 2 sticks vertically in front of, but slightly to the side of the camera on the tripod, and keep one about 4 or 5 ft behind the other.
Adjust the position of the camera on the tripod so that when you pan the camera, the sticks stay together and don't get further apart as you pan.
There's a good explanation and a video of it on the www.Acratech.net
Once you've done it a couple of times, it only takes about a minute to do.