if you think your stats need improving then yes take classes. During my year off I just worked to save up as much money as possible and applied w/ what I had numbers wise. Getting a job at a dental office is a good idea...it'll give you even more of an idea what the profession is all about AND help you make some cash on the side. But make sure you take time to enjoy yourself as well...I didn't and I so wish I would have taken a trip or something instead of working my a$$ off day in and day out. But oh well...you live you learn.
you can take upper level classes anywhere, just about. so long as you've completed the prereqs somewhere... you just need to apply as a non-matriculated student and be accepted. they'll either make you ask the professor permission to register, or just let you sign up w/o permission.
Do a 1 yr non-thesis biology masters program. It is hard to get work in a dental lab. Unless you know someone, they're likely not to hire you. They are running a business and want the people they train to be able to stay. If you want to get into dental school, a masters and more shadowing is the way to go.