The Q marker is treated as a sentence marker because its scope applies to the whole sentence. You can think of it as a device that triggers subject-operator inversion and ensures acceptable Yes/No Questions intonation (which we discuss below).
Notice that if this sentence had two auxiliary verbs-for example, if we were to add be+ing to the will in question 1- it is only the first auxiliary verb in the auxiliary string, the
Will they be gambling in Reno on Friday?
Operator, that is inverted with the subject. Furthermore, if the auxiliary has more than one element, as does the phrasal modal in question 2, than it is only the first of the element in the first auxiliary, again the operator , which, along with the tense marker, if there is one, is inverted with the subject. Here are the trees for question 2 as an illustration of this last point.
Was she able to finish in time?