Yes, "请问" can indeed be ambiguous, because it can mean
"may I ask ..."
May I ask who you are looking for?
or it can mean:
"please ask (someone) ..."
I don't understand this either. Please ask the teacher.
How do you know which is which? Well, as with pretty much everything language, context is the key. Context will make everything sufficiently clear. And if there is a need to add clarity still, you can always add the word "you" to the second example. 這個我也不明白，请你问老師。
And here's another "clue" you can look for in a sentence to help you tell the difference.
When 请问 is followed by a question, it mean "may I ask ..."
请问你認識他嗎？ May I ask if you know him?
请问這個字怎樣寫？ May I ask how to write this character?
请问您貴姓？ May I know your last name?
When 请问 is followed by a noun, mostly likely a person, then it is "please ask ..."
今天有什麽功課，请问老師。 Please ask the teacher what we have for homework today.
你想借錢，请问銀行。 (If) you want to borrow money, please ask the bank.
Answer to your other question: 大家 is not ambiguous. A big family is 大家庭 not 大家, as explained in another answer.