It depends on whether the sentence is counterfactual or not. If the sentence is counterfactual, you should use "If only". If the sentence is not counterfactual, you should use "as long as".
2a and 3b are obviously counterfactual, so you should use "If only".
I think your other examples are not counterfactual, so you should use "as long as"
is very likely to imply "If only we knew the address, then we could look it up on a map and go". Since it is using the conjunction of opposition だが, it implies the transportation is not really convenient.
In other cases, it's hard to determine if only ~ then or as long as ~then unless the reader knows the ...