I don't think MR kidnapped him. I've explained why before - obviously MR isn't with DR right now, and the only end-game for kidnapping will result in him going to prison. This holds even if some family members of his kidnapped Dylan. (All this assuming Dylan is alive of course.)
I agree that there probably would have been evidence around the lake if he was fishing; if he was fly-fishing then he probably would have brought a tackle box with him for the lures. It's possible that it all fell into the lake, or someone came by after Dylan fell in and took what looked like fishing gear someone abandoned, but I don't think that's likely either.
I think there are three possibilities:
- Dylan was kidnapped by someone (not MR) and is still alive
- Dylan was kidnapped by someone (not MR) and is dead
- MR killed Dylan
I think the kidnapping by someone other than MR makes the most sense with the evidence we have, as MR killing Dylan would almost certainly result in some kind of evidence left behind, and we haven't heard about it nor has MR been arrested.
EDIT: and I want to emphasize "with the evidence we have." If the police come out and say they found a few liters of DR's blood in MR's house, that will change my mind to say the least.