If Germany should realise that the Atlantic traffic is important, the more 'easy' one could be the transfer of men and materiel from UK to France - the Channel
Using limited resources (an MTB vs. battleship in cost) and focus on mining and MTB attacks evrey day could have an impact.
Not really. They'd not have anything remotely equivalent to say the S-Boat's of WW2 due to the technical limitations (they'd have to be either coal powered, which means big bulky engines, or use the limited amount of petrol available taking it away from aircraft.) Also they'd be very short ranged so would be of limited use.
The UK's not going to force its way past Denmark, the advantage of the Danes joining the Germans in WW1 is limited, their army was tiny, their available manpower limited and their industrial base also of little use, but the UK's not going to do that because of the perception it would have on the world stage. But what they can do is threaten to go for places like Heogland, and capture that off the Germans. The RN isn't going to go into the Baltic, its too close to German bases and many many u-boats though.