To answer your questions briefly--
1) It will depend entirely on the type of institution you end up in. If you're going to be a serious researcher, compete for R01 NIH grants, publish substantially, get tenure, run a lab, etc. you will have to expect to devote almost all of your time to that aspect of your career--like 80% or more. Research at this level--which is what MD/PhD programs say they are trying to train you for--cannot be "dabbled in" part time.
2) Yes and Yes.
3) To be taken seriously when applying to funded MD/PhD programs (and you don't want an unfunded one--believe me) you need to show that you can take on a project, produce some results, and hopefully at least be part of presenting it to the wider community via a meeting or publication. So find a mentor at your undergrad institution that has a track record of doing this with undergrads. Neuroscience is a plus--and face it, nothing else is really interesting enough to dedicate a career to, right?--but not necessary. Get started when you can, but you don't have to hit it right out of the gate.
4) Try to wash your mind of any romantic notions of what it means to be a physician-scientist. If this were easy, everyone would be doing it. Also be aware that medical school is a vastly different cognitive experience than graduate school--the former is essentially about filling your brain with facts and regurgitating them on multiple choice tests; in the latter you don't even know what the question is most of the time.
Good stuff--Vastly reducing your costs of medical school, possibly getting out debt free.
Getting to "go deep" in biosciences.
Getting to go to some cool meetings, hopefully publishing a couple of papers that will be sitting in libraries forever with your name at the top*.
Being a "hot commodity" on the residency interview trail.
Always having the clinical to fall back on if you can't make it in the world of tenure-track academia.
Bad stuff--Years taken out of the prime of your life spent banging your head against insoluble problems.
Messing with inane technical issues.
Watching your undergrad friends going on to have lives, getting jobs, buying houses, having kids.
Watching your med school buddies go off to clinicals, saving lives and s--t, while you bang your head against a lab bench or spend your weekends in a darkened microscope room.
Watching them go to residency while you're wondering if you have a defensible thesis so you can finish and switch into the rest of your MD.
Starting the whole process over in a research track residency, taking a postdoc-level research fellow position while you try to get a K award to start your own research, navigating academic politics, publishing or perishing...
*I still get emails notifying me when the paper that was the core of my thesis research gets cited. Not a lot mind you, but enough to remind me of my academic immortality.
