How much editing can one ethically accept from one's advisor on a fellowship proposal (e.g. NSF/NDSEG/etc.)

You apply for money and your adviser will profit from getting you granted the money, too. He will get some research he is interested. Why shouldn't every adviser support his student in that way?

You should also learn how to write proposals. Not just by writing it, but also to give it the finishing touches and fixing the issues you could not yet identify yourself. That's why its good to get the feedback and support you received from your adviser.

I don't see it unethical at all. It would be problematic if you could not perform the research you describe in your proposal or know it will not work or know someone already did it.