They can stay longer than 6 months on TRVs too by applying for extension to their visitor status.
If that's the case should I sju
To the best of my knowledge, CAN+ does not work for super visas, but only for TRV applications. But, their previous visits will help for faster decisions.
There are quite a few documents which you must submit as their host. Do check this link:
http://www.cic.gc.ca/english/visit/supervisa-how.asp
You can simply mention that they would like to visit you.
For super visas, your financial documents matter more as the host.
But, yes, they must prove they have strong ties+ finances to return home to because super visas are temporary visas unlike PGPs.
Yes, upfront medicals are accepted by CIC.
Yes, as many strong ties as possible for which they can provide evidence.
The insurance coverage must start from the date of their intended visit + they will need insurance coverage of minimum CAD 100,000 for each entry to Canada
Do check this:
http://www.tninsurance.ca/visitorscanada/supervisamedicalinsurance.htm
Cheers