Al,
#1. Take back the bottle stuff, it is expensive water (very dilute). The stuff in a bottle is a huge ripoff. Instead, go to Sherwin Williams paint store and buy the pure oxalic: 12 ounces will cost you about $9. FWIW: I buy it five pounds at a time from Van ****s (mail order) for about $12 as I recall. But you don't need that kind of quantity. Stop by Waynesville and I will give you some.
2. How long is all about concentration. I use the solution really dilute (so it takes longer to work, but costs less and is easier to handle: a concentration by weight of about 0.2% to 0.4%). At that strength, I let it sit for about 24 hours. And then I keep reusing the solution, several times, til I need the room and then I dump it. My current bath has treated at least ten frames.
3. Most rubber is pretty acid resistant. I would probably test something first, or just treat it briefly. Recognize that there are a lot of different materials that people call "rubber", it is not all the same for sure.
4. Why not naval jelly? Try soaking the inside of a frame, an entire frame or fork in naval jelly. I don't see a way it would work. Oxalic is cheap and effective. Why use something more costly and less effective? For small spot treatment, use the weapon of your choice: naval jelly or whatever.