You seem to be over thinking things.
Surely you know approximately the dates that your visitor plans to visit and you can find the approximate processing time from link below.
Others can comment as well but go to some online booking site such as Expedia if they operate in your country, put in an outbound and return date search. Select a flight for each leg and the site will show a flight itinerary before needing to book or pay.. Either print it or take a screen shot and there you have an itinerary showing a travel plan for 15 plus days.
You should not purchase any ticket before visa approval but in any visit visa application just provide a proposed flight itinerary as above and a schedule of what the visitor is planning to do on their visit, purpose of visit accepting that first 14 days will be in quarantine. Of course itinerary is only part of the visa application which also needs to include all the other things to show intent not to overstay, ties to home country and so on.
If the visa application takes longer to process than your submitted itinerary dates that should not be an issue given once a visa aplication has been approved the visitor is free to travel any time whilst the visa is valid. Obviously pitch the dates longer than the approximate processing times below.
It is possible at airline checkin in home country they may not be allowed to board unless they have a return booking 15 days or more in future so keep that in mind in addition to any travel restrictions for visitors that may still be in place due to COVID.
When the visitor arrives at POE they should be prepared to show to CBSA their isolation plan and also that they have a return flight in 15 days or longer than planned stay, although obviously the first 14 days would be the quarantine plan.
https://www.canada.ca/en/immigration-refugees-citizenship/services/application/check-processing-times.html