In the first and second examples, for two seems like to modify the previous noun phrase. But for the third, it’s not clear to me. It’s ambiguous whether it would be a modifier for the previous noun phrase or an adjunct/adverb for book. At first I read it as the latter. Do I have to correct my interpretation? Or can I keep it a right one?
I think your instinct, that for two is governed by the NP, is correct. As your first two examples show, a table for two/four/n is virtually a fixed phrase, signifying to the customer what the trade calls a "deuce".
The alternative would be to regard for two in example iii as designating the beneficiary of the booking. That seems unlikely to me; in fact, I can very well imagine an executive instructing her secretary
where the beneficiary is clearly me.
Welcome to the joys of context-sensitive English.
"book a table for two" is only spoken in the context of a restaurant reservation. The only numbers involved are the time and the people, so "for two" is usually 100% clear.
"table for two at two" is also 100% clear. 2 people (for), 2:00pm (at).
"For _" can also mean an approximate time:
"book a table for two pm". You will get a standard table seating 4. mid-afternoon is not busy at most restaurants, so they won't care about the number of guests.
"table for two for five" is 75% clear, only because the standard order is people first, then time. The restaurant will ask for clarification. Native speakers usually recognize the problem and choose different words.
"table for four for four" sounds like you have a speech disorder. Just choose different / more words:
"table for four people at approximately four o'clock in the afternoon"