From my knowledge of working as an ACMT, Apple processes every part it takes out of a computer during repair service. I couldn't say how many are fixed in the manner you're talking about, but I can say that a significant proportion of parts we received for warranty service were clearly 'remanufactured' parts. So that kind of toss-and-replace doesn't even really happen at Apple itself.

Maybe not (although Louis Rossmann does say that he checks and has many videos where he does), but they do typically at least check the connectors and the main components.

My brother has had a water-damaged macbook pro repaired by a third party repair shop when apple said the only thing that can be done is a logic board replacement and they fixed it without replacing it.