Separation of Responsibilities in a Scrum Team

We are now halfway through our second 2 week exploratory sprint. The team consists of 4 pigs:

1. Team Member, Developer, Scrum Master (myself)
2. Team Member, Tester, Product Owner
3. Team Member, Developer
4. Team Member, Tester

Both developers have fully embraced the methodology and the associated tools (TFS); however, the other 2 team members have greeted it with a less-than-enthusiastic attitude. Both seem to view it as an unnecessary overhead.

2 specific problems have arisen...

1. Product Ownership

The Product Owner (PO) has a very relaxed attitude to who can add items into the Scrum. To him this is sensible as we should all have an understanding of priorities. To me it makes everyone's job harder. I think I wanted the PO to have total responsibility for this area so that the Team Members (TMs) are free to just develop what's in front of them. This also removes any burden of 'why did you do that?' from the TMs.

2. Time Boxing

The PO is quite unwilling to defer tasks even though it is clear from the Scrum Burndown Chart that we are not going to finish all the work items in this sprint. Does this matter? I have tried to explain that one of the benefits of time-boxing and this trend data is that we can make decisions early and inform stakeholders and customers that certain deliverables will be delayed. This can now be done weeks in advance of the deadline (as opposed to the morning of the delivery!).

Hopefully, these issues can be addressed more fully in the retrospective.