1.) Will law schools look at the trend of my grades rather than just the cumulative GPA, since it is clear that when I got out of pre-med classes I started to truly excel? I'm worried that when looking at a 3.5 alone I will be thrown out at top law schools.
Yes, but tread lightly here. I would emphasize the trend, but you might want to soft-peddle that it was the change of major that did it. Will elaborate in a moment.
2.) Is this something to write about/explain in law school apps?
Yes. Same caveats as above. You will have a chance to write a personal statement and explaining improving grades and/or poor early performance is something many people write about. Also, some schools let you add an addendum to your application for this purpose.
3.) Most, if not all, of my extracurriculars are medically-related. Is it a negative to not have any law experience when applying to law school?
Not a negative at all.
Okay, here's the word of warning.
Law school does not require any particular undergraduate major. Unlike med-school, there aren't even any prerequisites. The folks who know they want to go to an elite law school have, in many cases, gone through their undergrad getting the easiest possible major. Frankly, if you KNOW you want to be an attorney, you're foolish to do anything but chose the easiest major with the maximum number of electives. You want to graduate with the highest possible GPA, period. Doesn't matter in what or for what.
Now, you, me, and all sentient mammals may know damned well that a degree in the humanities and a degree in biochem are about as comparable as a picture drawn with a crayon and the ceiling of the sistine chapel.
However, you need to take your audience into account, here. The admissions committee is probably a bunch of similarly brilliant JDs who may have fully understood how the game is played and got glorified basket-weaving degrees in undergrad. It's not that they're bad. It's that they understood how the game is played, and they played it. And they won.
They do NOT want to read how your grades improved when you changed your major from something hard to something fluff. That is saying, essentially, that the typical undergrad majors of most high-achieving law school students is a joke. It is also implying that these brilliant JDs from top schools would have mostly been lost in the sauce without a spoon if they'd had to take a real major in college.
At this point in their careers, they've drank the kool-aid. They will tell you up and down how their humanities degrees made them better critical thinkers, etc. Also, they will tell you that a humanities degree is not easy to get and that getting good grades requires brains and hard work (and they're right.)
I assure you this will NOT play well if you try to imply otherwise. At this point, these guys are almost all relatively wealthy, successful, and worked their butts off to get where they are. They are not willing or ready to acknowledge that their undergrad education was in any way inferior to anybody else's. To them it was just as difficult, just as rigorous, and it prepared them for the tremendous success they had in law school.
They honestly believe that their 3.9 gpa in sociology would have been a 3.9 in petroleum engineering. In a few cases, they are right.
So, my advice? Absolutely do NOT frame it as, "yeah, my grades sucked at first, but that's because I was taking a hard major with hard classes. Once I started taking the typical pre-law classes, my GPA shot up." The subtext of an argument like that is, "Well, I did okay when I was in school with smart people. But once I started competing with dumbarses, I whipped them like the morons everybody knows they are."