1) No benefit of booking QF flight number. As a jetstar operated flight it will not credit miles to your QFF account, and no difference in benefits.

2) Jetstar baggage allowances will apply.

3) Jetstar doesn't interline so you will need to claim bags in Melbourne and recheck with EK, and vice versa on the return flight. This also means you should allow plenty of time between flights - if JQ flight is too late you may miss cut-off for checking in on EK or vice versa for the return flight. If you misconnect due to JQ flight being late you'll be put on the next available EK flight, but that may not be the same day. In the other direction I suspect JQ will require a new fare to be purchased if you misconnect from EK to JQ.