But the fact is that most people playing White don't know the Jänisch that well and in the other hand the one playing always it as Black has a lot of understanding of it's principles and lines thatfore I would claim that in a pratical game it is more logical to use the Sämisch. It's same with the variation you pointed out in the Scotch game 5..Nb3 (The Carlsen's move) has always served me much better.
Whats the best way to defend in King's Knight Opening?

I think the Jaenisch gambit is a good idea at high level, but in lesser levels I'm not sure.
I couldn't disagree with you more. Openings like that are best when using them against players with low rating. For example I used to play The King's gambit quite a lot when I was weaker and won a lot. Nowadays when I play it players who know it better, I don't get any advantage. Openings that require studying the openinh or good understanding of chess are harder for those are weaker.

I don't think 2... d5 is reasonable. This is called the Elephant Gambit and usually leads to position where white has a great advantage.

My reccomendation is the portuguese... If 3.c4, you can play 3..c6!, and if 3.Nf3 Qxd5 4.Nc3 Qa5. If 4.f3 see the second diagram. The scandinabian is easy to play and learn.

[1.e4 e5 2.Nf3 Nf6 3.Nxe5] 3...d6! 4.Nf3 Nxe4 and black is already equal, and will have the better winning chances if both players are beginners
Did you tell all the grandmasters that insist on playing 1.e4 e5 2.Nf3 ?
Because if it was true, that means the Petroff refutes 1.e4 for all practical purposes. Allow me to doubt it.

[1.e4 e5 2.Nf3 Nf6 3.Nxe5] 3...d6! 4.Nf3 Nxe4 and black is already equal, and will have the better winning chances if both players are beginners
Did you tell all the grandmasters that insist on playing 1.e4 e5 2.Nf3 ?
Because if it was true, that means the Petroff refutes 1.e4 for all practical purposes. Allow me to doubt it.
Obviously black equalises in the petroff( see kramnik's games) but does so with more ease with the berlin defence.

[1.e4 e5 2.Nf3 Nf6 3.Nxe5] 3...d6! 4.Nf3 Nxe4 and black is already equal, and will have the better winning chances if both players are beginners
Did you tell all the grandmasters that insist on playing 1.e4 e5 2.Nf3 ?
Because if it was true, that means the Petroff refutes 1.e4 for all practical purposes. Allow me to doubt it.
Obviously black equalises in the petroff( see kramnik's games) but does so with more ease with the berlin defence.
How easier could it be than "already equal" at move 4 with no possible deviation for White?
I think the Jaenisch gambit is a good idea at high level, but in lesser levels I'm not sure.