[ls6]

On big backlog: you have surprised me here. My backlog has all kinds of stuff in it that I'm not even sure we will ever build (as I said before it is a research project). As long these vague stories are more than 2 iterations away they don't distract the team but, at the same time, show them roughly what I am considering in the long term. I personally find it very important that the team knows the big picture and where and why we are heading so they can take more informed decisions. As a bonus once in a while I get unexpected insights from the team. They are smart guys, after all :)

Regarding the client involvement, what you describe is, in my eyes, not your problem but your clients' - they clearly have no idea what they want to have. If the project is important for the client I sometimes add one more (senior) person to the project from our side and charge him with the task of helping the client to figure out their needs. This new person takes over the PO role on the team side but spends most of his time with the client asking questions, guiding him etc.