请问小白兔

First of all, a little bit background informaiton about myself: I got my H-1B (I797A) and the new I-94 for H-1B, which is attached to the bottom of I797A, last May, before that, I had always been holding F1 visa. Right now, the only US visa attached to my passport is still my old F1.
I will do my Canada PR landing next weekend, stay there for a weekend, then come back to US. And I will again go to Canada to get my H-1B stamp in May, I have a few quick questions:

(1) About the visa auto-revalidation policy, how can I prove I stay out of US for fewer than 30 days

(2) I will fly into Canada next weekend, do I HAVE TO submit my I-94 when I leave US? If so, I won't have my I-94 to show them when I come back?

(3) In case I don't get my Maple card in May, I heard I can drive a private vehicle into Canada without the maple card, is rental car regarded as private vehicle?

(4) If I do drive my own car in May into Canada, I read sth on this forum saying I will be requested to import my car, what does that mean? Do I have to pay tariff? Can I then drive my own car back to US?

Really thanks a million!