i landed by driving through Rainbow Bridge last year...ONLY BASED ON my experience
1)"我们打算先飞到西雅图,然后租车开到温哥华,开到美加边境时,是否只涉及加拿大入境的问题?"-YES;
"就是说,只见到加拿大的移民官处理入境手续,不会见到美国移民官处理出境手续"--correct.
2)离开美国的时候,美国移民官会收走i-94---No
3)don't know
4)yes
5)no idea about opt policy. I told the US officer I was on vacation in Canada.
6-7)don't know
good luck