Here are some of my own thoughts and some answers to your questions..
There is no way (yet) to repair aural nerve damage. I have a lot of hope for stem cell research, but at this point in time there is nothing.
I have found that hearing aids, both analog and digital, within their limitations, vastly improve what I hear. You will hear background noise amplified, but your brain already knows how to filter that out, and it will probably learn to compensate with the aids. In addition, the digital aids offer a programmable feature called "sound compression" so that really loud sounds get damped down. If the whistles and humming cicadas are tinnitus or "head noise" they will get drowned out by the aids; if they are actual sounds from outside, and you get digital aids they will get amplified only if they fall in the frequency ranges that you need to have amplified. If you get analog aids, they will get amplified no matter what, but your brain will learn to filter them out, as it does now.
Hearing, as someone mentioned above, fluctuates with several variables--atmospheric pressure, sinus congestion, earwax, etc. You should perform fairly consistently from hearing test to hearing test, but I imagine the standard deviation of such tests would, because of the variables, be difficult to determine.
When all is lost, and an ear is complete deaf, there is a procedure called a cochlear implant, which puts the electronics right inside your skull. It's my understanding that this is very expensive, and is only undertaken when there is no danger of damaging any remnant of hearing, because it is a very risky procedure.
Finally, my inspiration for most of my musical life has been an image of Beethoven, lying on the floor beside his legless piano, composing symphonies. However, three weeks ago I met a young woman with an 80% bilateral hearing loss, who has lived with this level of impairment since birth, and who has only analog hearing aids. She is the organist at a church, plays the piano, and conducts the adult and childrens' choirs and the adult and childrens' bell choirs. She says she can "feel" the harmonies and the music.
Finally, getting vented aids, either ITE (in the ear) or BTE (behind the ear) is essential to mitigating the "head in a bucket" effect and taking care of the air pressure and temperature differential issues. Some people have no problem with these things, others, like myself and Sir Roger above, get "irritable." That's an understatement, I think. The totally occluding earmolds send me through irritable, through cranky, to downright crabby and cross in no time.
I hope that's helpful. If it's any comfort, we're all going to suffer some circumscription of our abilities as we age...as infirmities go, progressive hearing loss is not, in my opinion, any where near the worst.