Cemented Main Drain and wall grate questions

As my pool knowledge and confidence grows - so does my ambition :
When we bought our house 7 years ago we were informed that the main drain in the pool had been cemented shut by the previous owner, due to a leak. These folks always took the cheap & crappy approach to problem resolution. The pool is a 12x24 (roughly) IG gunnite / plaster (I think) I'm kind of thinking of fixing the problem. I'm a bit of a novice at times - so here are my questions :
1) Do I need a main drain for anything other than draining? Will it improve circulation a ton?
2) How would I find the leak? Dig up the yard? What if it's "under" the pool?
3) Would I have to jackhammer the cement (I'm guessing) to remove it? It could be quite thick I'd imagine.

Also there's a small grate in wall of the deep end just below the coping. Maybe 5 or 6 " long and 2 inches high. There's a pipe attached to it - I can see the bend of a right angle pipe in the decking. I'm guessing it's to prevent overfill. Anyone know if this might connect to the plumbing?

Thanks.

-Rob