So if I were to use AP to travel to Canada what must I be aware of?
if you were out of status before, you cannot travel on AP.
Should I be turning in my I-94 (old, attached to the passport) and the new one that is part of my H1B ( 7th year)?
my suggestion is NO. especially if you travel by road. take off your I94 before you enter canada. They don't ask for it. however, if you travel by air, chances are they will take away your I-94. If this is the case, you can NOT use 30 days rule.
Will I be getting a new I-94 when I return from Canada to US if I used AP?
yes, you would get a new one if you use AP
Do I have to tell the US immigration the fact that I am travelling to Canada to land as a permanent resident?
Not necessary. don't provide info that is not asked for.
Applied for I-140/I-1485 from VSC in July. Both are now pending
Got my AP and EAD
I guess your I140 not approved yet. If your think there is chances your I140 could be denied, then you should be cautious to use AP. H1b is always safer. you could try get visa stamp if you could not entry with the 30 days rule. use AP only as the last resource.
just my 2 cents, consult your lawyer.