why would someone travel back in, and through, time, just to drop 79,950 feet through our atmosphere.
Well 1st why would you turn an event into a singularity when all things cause other things to occur. Droping through atmopshere could cause a domino of atmospheric events. However generally the reason you would want to get close to something is like a probe. Sensor readings, interception of communications that degrade, etc.. etc.. To hide from spatial sensors etc.. Tons of reasons. You could write a volume of books on that.
there is very little sense in even coming back to recent times.
We are actually making some big pushes in science and constantly on the edge of extinction with the destrcutive power that humans have now. In fact by interacting with higher level particles, we could actually be interfering with high technological science. Take for instance accidentally jamming radio communications while experiementing with electronics. You may not be aware you are creating harmful interferance to other peoples communications as you don't have an ear to that. Thing is while Terrans have a massive knowlege of science, fact is scientists are still trying to figure out how stuff works. We have a very small timeframe to understand things like gravity, and there are non uninfied science theories out there to explain some stuff that Terrans don't understand yet. Fact is we could have breakthroughs at any given time in science that could present us with the ability to engineer things that could pose a threat to other advanced lifeforms that could peak their curiosity in what we are doing. Its not as if scientists have never killed themselves experimenting with stuff. Radioactive materials, radiation and such are good examples of playing with things you don't understand. Now we are creating micro blackholes. If you really think about creating microtears in spacetime could always possibly create side effects, or interfer with other peoples technology. Releases of energy and energy matter conversion could probably alter earths position in spacetime. There could be lotsof reasons, but we don't know but if you send out a signal on purpose or not people may be curious what is going on with that.
Another thing that puzzles me is that the time-traveling vehicle would have to also travel through space.
spacetime is a singularity, again you are thinking about this as if time were a linear thing while paradoxial saying you can travel in reverse along that linearity. You need to rather think of it being a latticework of interactions that things are taking a form and every form is already expressed, and that forms are acting on forms. All you need to do is take your form and act on another form to intersect another point in time. Of course that is totally beyond the scope of anything terrans would be capable of right now en masse in fact it really is sort of insane, and when things get insane, it is sort of avoided.
If Earth is traveling at 45 million miles per day through space, the vehicle would need to make up for that motion/distance, I would think.
Again you are assuming space is linear/relative when in fact it is a lattice that interconnects and entangles across multiple dimensions of the lattice.
None the less science that works and able to be repeated is much more useful than science that is random and choatic and cannot be repeatable.
If you are a 4D entity, you cannot see beyond that but sensors allow us to see into worlds our human senses do not allow biologically (excepting extrasensory perception stuffage). Again don't limit reality to what your senses tell you exists. Reality has more to it than what terran biology can explain.