Well you've got your hands full..

How much do you run your system? I've seen inadequate pumps and filters keep pools crystal, but they ran up quite the electric bill. .

I know I'm not much help in this area, but I bet some one here will crunch the numbers. With a single 1-1/2" suction line a 2hp pump is a waste, but if your planning on doing some major overhauls then you'll have to plan for that. 72sqft sounds like overkill to me, a nice Nautilus at 48 should do the trick, don't you think? Or one of those new amazing cartridge filters. I was putting a lot of them in towards the end of my pool days (4 years ago). I didn't do much spec on the pools I built the salesmen and owners always did that. I just built the darn things.

Here's a thought though. Have you considered just converting one of the 3 returns to a "side wall" suction? Rather the trying to fix the main drain? Which tends to be costly and sometimes impossible. Or coring through the wall (not fun either). If you had 1 suction and 2 returns it's just robbing Peter to pay Paul. But with 1 suction and 3 returns, the numbers might work a lot more efficiently with a set up of 2 and 2. And the line already exists.

Is your pool free form, or is it fairly symmetrical with some good straight lines and well defined corners and angles?

I've done a couple vinyl conversions though. They're not fun and if the pool isn't totally uniform I suggest having the liner company measure it also. It costs extra but sometimes they will guarantee the fit when they do. I did a concrete wedge pool conversion once. Shear walls from top to bottom (deep end too). It looked like a car ramp turned upside down. The steps were free form and I had to supply cardboard templates of the treads (tops). I also had to take plumb bob measurements to make sure areas of the bottom didn't bulge too much. It was 8hrs just to measure the damn thing.Then tapconning the extrusion into the wall, was a ball breaker. Needed pipe staging for the deep end. Fit beautiful and the owner loved it. I believe we spray tacked foam on 100% of the pool, if I remember correctly. The liner company couldn't or wouldn't guarantee the liner with out some buffer in between , because of the roughness of the concrete. The only other option would have been to diamond grind the whole bottom and walls.

Just tossing some thoughts out there.