Some sources seem to claim that you will find more rare pokemon the higher your level. If true, would walking together with a higher level trainer improve your chance of finding more rare pokemon?
I don't believe it does. The Pokemon locations are shared among all users. It would be interesting if the spawns were dynamically generated based on the level of users in the area but somehow I doubt that. Anecdotally, I've seen roughly the same amount of rare Pokemon at level 5 as at level 15.
You are correct that the higher level you are the more rare Pokemon you seem to be able to find, but as for simply being near a high level trainer I don't think you'll be able to see all the same Pokemon as them.
The only source I have for this is personal experience when I was able to catch a rare Pokemon and told someone nearby who was new that day exactly where it was and they couldn't see it. (Although this could be caused by it "despawning" of sorts so I'm not 100% certain.)
Yes, however it's not only linked to what level they are, but also how many people you are walking with. I don't know the exact algorithm, but it is based off of both level and amount.