Probably not. Lots of Canadians have US Bank accounts. Those that lived there in the past often have investments there. As long as you can demonstrate the usual ties to Canada it is unlikely to be a problem.
No. If anything it would improve your application since you are not a broke tourist.
US is one of the biggest tax heavens in the world for foreigners. Millions of foreigners work with US institutions. You're not the only non-US person who visits US while having a US bank or broker account.
Do tell the truth, but don't give more information than is necessary.
So in the application she should call you her boyfriend. Trying to pretend you are not risks getting a ban for deception. Tell the truth about your actual plans.
However don't talk about possible plans that you might have on the visa application. Plans can change. Concentrate instead on ...