Thrush,
The sunwings deal is a two way street. Rumour (take note of that word please) is that up to 100 EU crews (that is 200 pilots) are needed by Sunwings this winter - being the peak holiday period in Canada. Those pilots will be sourced from (possibly among other companies) TUI and Viking. In the case of Viking they also take two 737-800s which in the UK winter season would be a drain on the company when work in Europe is slack - they also take some 737s from TUI but I am unsure of how many.
This probably ruffles the feathers of some Canadian pilots, as it does yours, but the result is to give year round employment for pilots from both sides of the Atlantic who might otherwise find themselves out of work in the UK winter/Canadian summer.
It also gives Viking flexibility. Rumour (that word again) is that the three aircraft referred to on the Viking airlines website to be grounded are the Sunwings aircraft which will be returned to Canada two months earlier than planned, thus possibly avoiding job losses amongst Vikings permanent crews.
As the Canadian pilots are employed by Sunwings, which is a Canadian company, they may not need work permits as they are not employed or paid in the EU.