"Excuse me, how many of you?"
Can we just replace this question? The English is really unnatural with no context. Even with context it would be pretty unusual. If they have to use it, wouldn't "How many people?" be better?
I got this question in the restaurant lesson, so it makes sense in that context at least.
Is it just me or are the sentences sort of strange in this section? Wouldnt this be 请问几位人吗? Can measure words be used without the noun?
Using 位 implies that the sentence is about people.
吗 is only used to make statements yes-or-no questions. It doesn't have to be used when the sentence has a question word (什么，几，谁，etc.) or when the 是不是 structure is used.
Yes, that is true. You can also use 个, which is another people counter.
Although you may know, there is a difference that worth mentioning.
位 is a honorific equivalent of 個 - so they are not interchangeable, or you would have appeared rude or too formal. This sentence is typically used at the reception of a restaurant, where using 個 to refer to guests is normally not acceptable. In reverse, it is not appropriate to use 位 when you are counting people among your family and friends.