Is it grammatical to say: "to ask [insert name] questions?"

I know you can say:

To make sure we had plenty of time to ask her questions, we took the morning bus to her house.

How about:

To make sure we had plenty of time to ask Mary questions, we took the morning bus to her house.

Is that a valid grammatical short form of: To make sure we had plenty of time to ask questions to Mary...?

Nothing wrong with either. The only difference is your audience. If they don't know who Mary is then her name isn't required. However, if you are talking about many different females in one instance, you may actually need Mary to clarify.