Hi
Normally it's 50/50 so if you stay 6 months, you leave for 6. Otherwise you are not really visiting anymore. What you can do is if you get a TRV for her that you can apply to get it extended every time it's about to expire. Say you have already applied for her sponsorship and that she would like to stay and wait. You of course can support her financially or she has funds she can show so that will not be an issue. I have heard of other people doing this. I don't see why it would not work for you.
They won't issue a TRV to a visa exempt visitor. She would be admitted as a visitor for 6 months, and could apply for an additional 6 months, but any requests for over that year may be problematic.
PMM