Well..., no: האדם היה כאחד מִמֶּ֫נּוּ Gen 3:22 the man has become like one of us and so on. Rabbinical Hebrew on the other hand could differenciate by using the form הֵימֶ֫נּוּ from him, taken from the Babylonian tradition. The doubling of the Nun stems from the addition of the suffix -nu to the doubled min-.
"You learn from us?" and similarly formed questions are acceptable and proper depending on context.
Your example of "You looking at me?" has a different form so it does not support your argument. With the "ing" ending as a present tense verb you would usually say "Are you looking at me?"; however, even that would be understood informally, such as the well known De Niro quote in Taxi Driver "You talkin' to me?" You can use the same form as JeffreyG.R's post, and say "You look at me?", which would be correct depending on what is being empasized: "Do you look at anyone?", "Yes, at you.", "You look at me?"
This one's a little confusing for me still too. But I know the preposition itself is 'מן' meaning 'from'. For example, "I'm eating from the plate" would be "אני אוכל מן הצלחת". As for the various contraction endings, perhaps re-doing the second lesson of Prep1 will help, since that appears to be the lesson where they're taught. Looks like the –מ prefix is for 2nd/3rd person plural, –ממ is for 2nd person singular, –ממנ is for 1st and 3rd person singular, and מאיתנו is the odd one for 1st person plural. Someone please correct me if I'm wrong.
Yes, the interrogative particle הַאִם can only appear in yes/no question or choice questions (הַאִם אַתָּה טוֹב אוֹ רַע are you good or bad?). It cannot be used in wh-questions combined with question words like מִי who or לָ֫מָּה. Tag questions like xxx, isn't it? use נָכוֹן or לא.