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I like the idea of sending affiliate links through a redirect on a subdomain because you can kill off all of those links at once by removing the redirects so they don't affect the final destination page should you need to clean up your link profile of page-rank-passing affiliate links. Whereas, if you had those links go straight to the destination page you'd have to ask the affiliates to remove each and every link, and/or use the disavow tool on them. I have heard of people sending those though a URL shortening service instead of a subdomain, which should work just as well since they are 301s. I'm not sure how much sway keywords in the redirecting URL have, but it isn't a bad idea to keep them contextual as you described above. That just seems to make sense anyway. Since the affiliate.subdomain URL is going to 301 redirect it won't have a canonical tag. You could put a self-referencing canonical tag on the final destination page if you wanted to. The number in the URL is fine. This seems like a good topic for discussion so I will leave it open for you. PS: My answers assume you know the risks of passing pagerank through affiliate links and are willing to take them.

Social networking such as replying, liking, updating sites. Forum posts from good quality sites. Not a proper answer just a few ideas that jumped into my head!