Hi there.
1. Great effort on your part, but sometimes due to geography, the leecher's ISP speeds, leecher's client port forwarding configuration, etc, you will be unable to upload to that requester directly due to network limitations. It is largely out of the seeder's and leecher's control.
How long should a reseeder continue on for ? - It's entirely up to you, as it is entirely based on goodwill. There is no penalty or reward for fulfilling the uploading request, and as far as I know, there is no practical way of tracking it (i.e. which specific user seeded what and how much to specifically whom), nor can I see why a tracker would want to keep logs of this on every single user - the data would be enormous and quite useless. Continuing on from my initial point, there is therefore no guideline for how long a reseeder should continue doing so.
2. Again, I would base it on goodwill. Those will low ratios will have to find ways to get their act together (upload a new file, use freeleech only, etc). I think it is pointless to judge others, as each user has unique circumstances and there is no use in assuming anything.
Ethical ? No comment. Is torrenting porn taken from somewhere ethical in itself ?
Your duty ? No. It is but a request.
Best of our ability ? Yes. Do what you can.