Hello,
(1) The upload requests come directly from the participant’s browser, so it will come from their own IP addresses (the request will include an origin header indicating where the experiment is being run, ie farm.pcibex.net if you run your experiment on our farm)
(2) The address will only be constant as long as all the participants take the experiment using the same connection, which won’t be the case unless you control how you recruit them (for example by having them all come to a lab and use a computer there)
Jeremy