The rule about using only a single cube per action doesn't apply to these mixed-effect cards from the expansion.
Right. This is spelled out at the bottom of page 1 of the rules.
Another key point is the following.
You place cubes on Activity cards with a delayed effect in the original Troyes game so as to benefit some other Action in the future. Each of these cubes lets you modify the dice group you are using for such an Action.
(This is also true of the delayed-effect Activity cards added with The Ladies of Troyes.)
Placing cubes on a mixed-type Activity card in The Ladies of Troyes is different. These cubes don't affect the dice group of some other Action later on. Rather, they can be used only for later activations of this same Activity card (as Todd pointed out). Such a later activation needs its own dice group (as well as the cube or cubes on the card).