- Exact dates (in and out of Canada) - They have been coming to Canada for only a couple of months at a time - since 2010
- If there are some specific reasons why they did not stay in Canada - They would come for very short periods - two or two months in the summer, the reason for not staying longer is because of taking care of their parents and they can only have a specialized nurse for just a couple of months at a time.
- When they would realistically be able to come to Canada - They can come as soon as possible because they have now a specialized nurse to take care of their parents.
- How long they would plan to stay in Canada - 6 months each year
Is it worth it for them to fight for it - or to simply renounce it - and apply for visitors visa (while also going through the sponsorship process again?)
If they were to get the PRTD/PR status, there's a high probability they would effectively need to remain in Canada for two years straight to become compliant with the residency obligation, and have problems departing and returning as they seem to intend to do.
This doesn't really seem to be part of their plans - just temporary residency regularly in Canada to see their son. I'm assuming no expectation they will work in Canada.
Their plans don't really correspond to 'permanent residence' in Canada. They would quite possibly in future have 'unexpected events' come up that would also make maintaining compliance difficult. Such future events may mean a repeat of the current situation.
So personal opinion only: I'm not convinced it makes sense for them to 'fight for it' (and it may not be successful and/or be costly). They don't really want to become permanent residents, established primarily in Canada.
That said, there are plenty of other aspects I do not know about their case and their needs, and I've made some assumptions which may be wrong. And there is no guarantee they will get a long-term TRV or similar after they renounce. So again, personal opinion only and not advice.