Particular attention should be paid to application of mineral wool boards - why? Because the contact of the adhesive with the surface of the boards, as a fibrous material (often hydrophobized), has to be strengthened by tipping a thin layer of adhesive into the place where the base layer of the mortar will be applied.
It is worth to answer another question: should the contact layer of mortar have a different consistency than the base layer?
There is no need to dilute the mortar, and there is no need to wait until it dries. The contact layer and the base layer of the adhesive should be bonded together in a 'wet on wet' method to increase the adhesion. Nevertheless, there are mineral wool boards which are factory-primed with a special tacking agent. This kind of board allows the plasterer to skip the initial plastering of the panels - but before you decide to, make sure you are using the boards indicated above!