I might differ with your statement that "you don't need too good of hearing to conduct an orchestra". I'm not sure what you base this on or exactly what you mean. ASSUMING that the musicians are of high enough caliber to stay together in tempo AND play in tune with each other (obviously Beethoven would have been incapable of knowing if either was the case), all should go fine. But one can be conducting, even being able to hear, and still have trouble keeping a group at the tempo you want. Plus not being able to hear results in your not being able to gauge and regulate the balance between the orchestra sections, and so being able to control dynamics goes out the window as well. Intonation is relegated to the musicians being able to "tune as you go" as the saying has it. For intricate rhythmic passages in the music, you would have to rely again on your musicians as possessing a very developed sense of being able to listen to each other and stay on the beat. So while Beethoven indeed had to be able to hear everything in his mind, his aural conception of what he heard and what the audience and musicians heard could well have been vastly different. Undoubtedly, as you say, his being able to see certainly must have helped, but there are still a heap of variables that are not within your control if you can't actually hear what is going on. I'm not sure what you mean either when you say "that era depended on the composer being able to communicate the music accurately in classical sheet music, so that it could be followed". How is it different now? Present day symphonies still rely on written music notation and the skill of the director to bring forth the intentions of the composer. It always strikes me how the development of digital music software and production, effects devices, equalization, and the attendant technology that goes along with all that is basically meaningless when it comes to what a band or symphony orchestra does. Presenting a concert of modern or older band and orchestral music remains basically unchanged over time in terms of how the musicians play the music and how it is rehearsed and conducted. I taught instrumental music for 34 years, and it always struck me how - even though technological advancement changed the lives of public school teaching greatly during the course of my career - the clarinet instruction I gave my students in 2011 when I retired was virtually unchanged from the instruction they got in 1978 when I started! Same instrument..same fingerings...same embouchure, tone quality goals, articulation...on and on. A quarter note in Beethoven's time is still a quarter note today. Similar concepts can be applied to the workings of an orchestra using acoustic-only instruments. While some things change, some things remain pretty much the same.
Thanks for your thoughts...I enjoyed reading them. My favorite statement is your last one..no substitute for being able to hear! I might venture one step further and say that there is no substitute to hear tinnitus-free. And in terms of digital mixing and mastering, it is indeed a different world. One can only imagine the torment that Beethoven endured. At least it might have gone better for him if he didn't find composing so difficult!