Church scholars would say that Calvinism doesn't reflect the theology of Calvin, that Lutheranism doesn't reflect the theology of Luther and, as you say, Adventism doesn't reflect the thought of EGW i.e., regardless of what EGW herself thought or said, she has become a de facto "inspired interpreter of Scripture." Difficult to understand just how she evolved into that. During the controversy over the law in Galatians, she was unable to even give her opinion of the matter because she couldn't find the manuscript recording what God had "showed" her. She despaired over the use of her writings during her lifetime, as she saw how they were abused and misunderstood.
Whatever Lutheranism has become today, at ground level, comparing EGW with Luther, each had a different emphasis. For Luther, justification by faith was the center of his message. For EGW, sanctification was her burden, which explains how the door was open to perfectionism/LGT, a teaching eschewed by Luther.
EGW explained that Luther considered the Second Advent hundreds of years away; therefore he couldn't have prepared people for the Second Advent. Problem with that is that Luther didn't teach that, at least not consistently. His "Sermon on the Last Days" is replete with examples from contemporary times indicating the nearness of the Second Advent.
As long as the Lutheran/SDA alliance remains superficial and convenient, there shouldn't be a problem.