I had more than enough credits to graduate in December when I was in college. But I stayed for the full 4 years. Why? Because I got to take lots of 'fun' stuff with the extra time! I knew by the end of my junior year that I was more than fine credit wise so I gave myself a slightly easier senior year. I took some harder classes but also stuck in some more 'fun' things (like an Acting class) so that I could have a little break from straight studying.
If your daughter is taking lots of hard science courses (which it sounds like she is) encourage her to play with her schedule for senior year and find something she might have a small interest in, or some random class that just sounds cool and mix that in with the harder classes.
Even now there were even more classes I should've considered taking (but sadly didn't) and I regret that. I'm sure she can find more than enough to make her busy next year
. As long as she can afford to be in school a full 4 years, it's better.
And as far as a January start thing. Some grad schools do not let you start in January due to when classes are offered. I got a Master's in Social Work and a Law Degree from the same school. The School of Social Work allows students to start in January (although I started in the Fall) because the core classes are always offered (partially too because some people only go part-time). The School of Law however has a very set schedule for 1L year and does not allow January matriculation for that reason. They don't even allow part-time students (except under incredibly extreme circumstances). So start times can vary not just by school but by program as well.
Good luck to your daughter in whatever she chooses to do!