Perhaps I am over-thinking this however, would Oct 30-Nov 10 be too late for foliage colors in Nikko ? Will it be too cold in Oku-Nikko for hiking as opposed to mid October ?

I am trying to be in Tokyo and Nikko when nature is likely to be at peak colors ! I know we can't be assured of anything but looking for best odds.

From what I saw, they weren't short of cars so in that sense, yes, you could probably pick up a rental car by walking in. Whether you could actually complete the transaction is another matter, however, that would depend on their command of English (nonexistent, in my experience), or yours of Japanese. I used http://www2.tocoo.jp/en as the booking agent: they have an English website and decent rates. Then you just turn up and the paperwork is all ready. I've used them several times.

In Nikko, the actual rental was from Nissan, who are opposite the Tobu station. Nippon rental is across the street. You must have an international license to rent in Japan.