Overflow on pond size has been crimped down to approx 4" diameter now too we find. This overflow pipe was damaged by neighbors who used the pond while it sat empty several years we found out, and at one point a fellow told us he hit the overflow pipe so hard it bent/moved to the angle it is at right now, which dropped water level 4-5' from where it normally was. I took many depth measurements from my inflatable boat back in November and seems we have approx 8' depth all around right now.
We want to raise this level up to where it used to be.
Despite your information-packed post, I still don't think we have enough information to answer your questions.
Was the inlet of the overflow pipe bent down 4-5' below its original elevation? If so, then you cannot raise the water level until you fix that problem. Or is water entering the damaged pipe BELOW the inlet, or BYPASSING around the outside of the pipe?
If the 10" corrugated metal pipe is 50-years old, then it is almost certainly time to replace it. The most common procedure would be to drain the pond, dig out the old pipe, install your new outlet (of whatever type) through the dam, and then repack with good clay.
However, it sounds like you have a good fish population that would be ruined by draining the pond.
If that is the case, then the second best option may be to plug the discharge end of your current outlet pipe and fill it with cement. Then install a siphon system through the upper portion of the dam - which can be accomplished with only a partial drawdown of the pond.
Good luck on your project!
