What is a Prospect Merchant at DataFeedFile.com?

A Prospect merchant is simply a merchant who has not given DFF their official blessing. Whenever possible DFF would like to be recognized properly by the merchant.
A merchant can avoid this Prospect status by properly signing up with DFF using this link: https://www.datafeedfile.com/join_merchant_signup.php

Can Affiliate use Prospect Merchant data feed?

You (affiliate) can still use a Prospect merchants data feed on DFF as long as you have the approval from their affiliate program.

Would Affiliate get credit for referral from Prospect Merchant?

Yes, as long as you have an approved relationship with that merchant at whatever affiliate network they are tracking at.