Well, the prescriptive argument is that אֵיפֹה is composed of אֵי פֹּה where + here and that אֵי does not work well with the one-syllabic prepositions בכל"ם. This is a blatant example of the deep difference between a puristic rule and actual usage. In the Bible the form אֵיּ־מִזֶּה was used, i.e. you could only add the direction after this adverb of place, like אי מזה באת ואנה תלכי Gen 16.8 where did you come from and where are you going to?
agreed--when I was in Israel the question was always, "מאון אתה" granted this was either in a professional setting, or most often in temple where I was something of a Christian oddity. Also it was in "archaic" Jerusalem and they may have been using their best Hebrew on me. In my "modern" Hebrew course from Brandeis University--they would not have accepted מאיפה for national origin, but would if you were coming from someplace local.
You're right that בא is used in way more contexts today than it did 40 years ago. But whatever grounds it gains in various metaphorical senses, it loses to מגיע in the simple sense of "come". Classically בא=come and מגיע=arrive, but recently everybody say מגיע regardless.
Yes, בא לי is slang, very very common for decades, especially dominant with little children. It means "I want", but hints at a whim rather than a well-grounded or deep wish. It's origin is an amazing coincidence: as you say, it literally means "it comes to me", which fits the idea of a whim that "came to me out of the blue". But it actually came from the Arabic expression "jai 'ala bali", which means exactly the same ("I want", suggesting whim) - but the Arabic expression literally means "comes to my mind", and the "bali" is actually the "my mind" part.
Why are some spelling mistakes "acceptable" with just a correction and a notice that you've made a spelling mistake and other times you're marked wrong? I find this very frustrating! A simple spelling error from listening to the speaker shouldn't be marked wrong necessarily.