There is some great info in the posts above!
Reading your post I thought I'd share what works for me with you,attaching the leader to fly line-I feel that a store bought connecter of any sort will give you alot of drag,may not effectivly transfer energy to the leader,and possibly cause a weak spot(as you have seen).
Personally I use 20lb. test mono, and attach a ten to twelve inch piece to the fly line via a nail knot.Then I tie a perfection loop on the other end of the mono,and attach the leader(I like 9ft knotless tapered) with a loop to loop, and then add tippet via blood knot.This way you can use a leader of your choice, without cutting your fly line each time you change,and Imo a perfection loop is easier to tie than a blood knot on the water(a good thing IMO).
Fishing indicators...I try to fish indicators as if the indicater were a dry fly, if you can dead drift your indicator the same as a dry fly, there is a good chance it'll impart less drag on your fly.
I'm by no means an expert, but as I said this works for me.Hope it helps