Landing & PR status questions

1. On H1 in US with family on H4 - latest I-797 but old I-94 in passposrt, can we land in Canada, come back without any problems at US side - INS. Someone tells me there's auto revalid cluase
2. What's the best way to land and return, if not immediately planning to move to Canada - should we take Bus/Train/Rental Car. Can we go public transport and return using rental car! is it easy/cheap to rent car in toronto!
3. Do we need to go to canada every year to maintain status or is it fine if we go at end of 3yrs. from landing & spend next 2 yrs. in canada (can 2yrs. be in one strech or have to be broken up!)

Thanks very much for your advise and experiences