Implications of high school GPA trend aren't worth examining... like you said, they matter little. I also wouldn't generalize BS/MD programs because they vary widely. The bottom line is that there are such a wide variety of factors that go into college admissions because there are so few standardizing opportunities (ACT and SAT are pretty much it).
For example in my school, kids from the top 20% of the class got accepted to BS/MD programs. No where near perfect grades, little exposure to medicine, ACT scores below the 90th percentile. If the question is as simple as "does an upward trend look good," then the answer is yes (of course it looks good). But you can't give practical advice from a "ceteris paribus thought experiment" because all things are never equal.