Do I need to suplimentary seal the joint between pavers and patio edge when that joint is used as a drain channel?
Leaving joists between two different elements and/or two different materials without any seal is never a good idea if you ask me, especially outside. Even if everything works great it can look a bit ugly because elements could be in different slopes or slopes might be in different planes and some edge differences between elements might become visible which doesn't look good. The other reason is water can get into these gaps and cause many problems. So to put it in one word yes, you need to seal the joint. Next question is which material should be used? I don't recommend any type of batten for so many reasons which I wouldn't like to explain here. I would recommend some type of caulk specialized for outdoor usage. Any half decent seller at any specialized store can recommend you such product. Also you do it old way: back in early days when there weren't tons of specialized products people were using bitumen for this purpose. Maybe it is not going to last forever but it will get the job done and it tolerates temperature differences, at least for a while.