1) If she attempts to come back to Canada before/while we're applying for immigration and she is denied access for whatever reason, does this interfere with the immigration application and if so, to what extent?
It doesn't interfere with it at all.
2) If she has spent almost 6 months here, goes back to the states to reset the 6 months and heads back here as soon as a couple weeks later for another extended visit, is this susceptible to cause issues at the borders?
Officially, you are not supposed to be living in Canada more than not on visitor status so they could refuse entry if they realize she is coming back after only 2 weeks away. Like jayde said, you can also apply to extend her visitor status without her going back to the US but in order to do that, you need to know when she entered Canada so she either needs to have a passport stamp or a visitor record. Does she have a passport stamp for when she entered? The instructions to apply can be found here:
http://www.cic.gc.ca/english/visit/extend-stay.asp Give as a reason that you are about to apply for sponsorship. Show that you have the money to support her.
3) She doesn't have a job and doesn't have a lot of money, but she still has her name on a lease in the States. She would also be travelling by plane with a two-way ticket (from the US, to Canada, and back to the US 5-6 months later), with minimal luggage. Is that sufficient to proove she has ties in the States and is not trying to move in Canada prematurely, or can we expect extra questions and potential trouble?
Two way ticket is good. Not having a job can be a problem if they start asking.
4) While crossing the border, is she better saying we got married this summer and are planning on applying to Immigration Canada as soon as we receive the last official documents we need (and bring the immigration application fee receipt with her, as well as pictures of the wedding and stuff), or should she avoid mentionning that topic? Keep in mind she will have been in canada for most of the 6 months preceeding her reentry here.
If she is asked, just tell the truth, that is best. They appreciate when people tell them the truth. If she is not asked, she doesn't have to volunteer any information. I have never had any problems when entering any country and I have never once volunteered any information. Just tell them the truth if they ask something.