Clotty! I am little bit lost here. Why would someone surrender the I-94 at the departure point. My understanding of I-94 is that it is status prove document, not a travel document. I have travelled once and I left I-94 at home. I took RTD, and it was as good as Passport. No one asked me any other document. My route was from Seattle to Newyork; from Newyork to Ethaopia via Italy. and from Etho to Kenya. and on my way back I took the same route. No need of I-94 and surrendering it; however, I don't really know about short term travels like going to Canada which need Greencard.
The only people who I am aware of who need to surrender their I-94 are people who are issued a short term I-94 and their I-94 expired and need to go back to their countries. these individuals need to surrender their I-94. Also, if you want to abandon your status you can surrender your I-94.
Note: My purpose here is not to discredit a point or prove one. Just to learn. So please don't be offended...
Your last question: Again Greencard is not a travel document. I don't even think you would need to take it with you if you have a travel document. The law says you should have your greencard with you at all times, but I have seen many people who travelled and left their greencard at home and no question was asked.
Again I have a problem with understanding th confusion here between travel document and other status documents...