OK:
1. The pic isn't super clear (it might be easier if I photo the pannier I have by my side), but you see the strap in the middle at the top of the bag? Once you have rolled up the top, that then tightens across the lid (clipping just out of view on the pic), so you can use the should strap without it opening.
2. This is a limitation of the design – in essence it's open or closed. I suspect that it really depends on how you use the bag. For me it's a sealed bag or it's open and I'm rummaging through. What it doesn't permit one to do is have the same ease of access as a regular clipped lid would. But, that's the price to pay for 100% waterproof-ness.
Let me know if you want pics and I'll incur weird glances in the office and post up..!