Question. When I get in line for tickets for the ET, do I have to use them at that time of purchase or can I request a certain time to come back to the ET later that day or evening?
I believe tickets for the Eiffel Tower are time-specific, but it should be possible to ask for a specific time at the ticket office and then come back later. I'm not sure that would be worth waiting in a queue for, as it would be a lot easier to book the preferred time online.
I do not know the answer, but I do know that many places will not allow you to book a reserved time ahead when you stand in line, so don't assume you can. It is well and good to suggest booking on line but since these tickets are routinely gone in less than 5 minutes once available, that is for most tourists not really possible.