I'll try to answer your questions:
1 - Yes, as from Q1 2011 the RadTileView supports UI Virtualization
2 - Again, from Q1 2011 you can customize the size of the RadTileViewItems with the RowHeight, ColumnWidth, MinimizedRowHeight and MinimizedColumnWidth properties. You can also take a look at this example.
3 - This behavior is not available out of the box, but you can achieve it by editing the template of the RadTileViewItem and doing some checks in code behind. Could you please examine the attached project and see if this is what you are looking for. The drawback is that you can't drag and drop items if you don't have a header.
I hope I was able to help you and if you have more questions feel free to ask.
the Telerik team