I've done water bottles on my shoulders before. I found that I was comfortable carrying a 20oz bottle of water on each shoulder, but the 1Qt bottles where just a bit too big. I normally use the powerade bottles if that matters. Depending on where you hike, going SUL is a great way to simply carry less water, so this may work for you.
I've put a tarp out the outside of my pack using lashing plenty of times, and I haven't really had a problem with it
As for a removable pockets, I would think about doing something like Example 1 as the easiest. If I were doing it, I would just make a channel at the bottom and string some drawcord through it. Tie the drawcord to the two bottom lashing loops. Then, making a channel at the top and put some shockcord through that. Tie the shockcord to the top two loops. This way the top opens up, but not the bottom.