(1) 2 …g6 3 e3 Bg7 4 Be2 Nf6 5 0-0 0-0 6 d3 c5
Here I play 7.Cc3. I wrote a technical note (sorry, it's in french) about this move here :
http://ouverturebird.free.fr/fichetechnique11.php (2) 2 …Nf6 3 e3 Bg4 4 h3
I disagree with linksspringer here. Maybe I am wrong but I think Danielsen never played 4.h3 because on 2...Nf6 he plays 3.g3.
And I don't know why but there seems to be a big difference between playing 1.f4 d5 2.Nf3 Bg4 3.e3 Nd7 4.h3 (Danielsen variation), and 1.f4 d5 2.Nf3 Nf6 3.e3 Bg4 4.h3. In the first case I always had a good game after h3 and in the second case ... well ... too many losses.
(3) 2 …g6 3 e3 Bg7 4 Be2 Nh6!?
I played this variation once with white against a 2150 and I didn't have any real problem after 5.0-0 0-0 6.d3 c5 7.e4!? Nc6 8.c3 d4 9.h3!? f5 10.e5. But against a stronger player it would certainly be more difficult.
(4) 2 …Nf6 3 e3 c5 (5) 2 …Nf6 3 e3 e6!?
Sure that's a problem. Here I play 4.b3, Bb2 waiting for ...Nc6 from Black before moving the Bf1. But if Black play 4...g6 it may lead to an equal variation. But fully playable.
In case (4) I played once 4.b4?! but i think it's a rather bad move.