No, honest peers can easily ban them for relaying invalid transactions, all the way to the mining level. You don’t want to run the risk of cutting off your route. Also, tx validity can be processed in parallel with the ECDSA sign operation, so I don’t see this as a major practical optimization, though this should be tested. Would love to hear arguments otherwise though.
Not sure I agree. At the end of the day, miners need to get paid by including valid transactions in blocks, so they should absolutely ban peers that open them to a DoS vector.
They may, because they can’t tell if the fee will be paid unless they are holding state. This would open them up to a trivial DoS vector, especially with peer banning.
Full nodes serve as relayers and get paid. Eventually, this role will move to proposers.
This is a way to incentivize full nodes with “proof of resources” in Ethereum. There are other incentivization schemes that will incentivize making blockchain data available (serving downloads to other users who are not storing state). This will of course make downloads cost money. I’ll be proposing such a scheme in the next few days, though some proposals for this already exist.