1) Is Keldeo able to use Secret Sword only in its Resolute Form ?
2) Can Keldeo remain in its Resolute Form as long as the player wants ?
I'm asking beacuse I don't have a Keldeo (But I really want one :D)
No, he can use it in his Regular Form. Just take him to the Moor of Icirrus in B/W instead of Pledge Grove in B2/W2 to just teach him the move.
Yes, as long as he knows Secret Sword.
Source: Experience & Bulbepedia/ Secret Sword