The use again of the 757/767 fleet will depend on what stage of engineering requirements are needed - TUI although they may own that fleet they will likely not want to pay out for any costly Checks that maybe due. The 757's were planned to have gone by now but have had their grace extended a couple of times due to the MAX debacle.
As for the Sunwing Canada leases - that contract has been around for years, and is possibly a lengthy long term one.
The summer/winter arrangement suits both Companies and a cancellation penalty would be very costly.
As for the recent 738 purchases or leases to cover the MAX groundings. Perhaps TUI have also taken into account the removal of the 757's by procuring these.
As for TUI's mammoth expansion plans for W20/21 and S21 I cannot see TUI throwing caution to the wind and staying with that plan.
Once holidays restart (who knows when) there will be a huge rush for folk to travel to the sun if they can, however restrictions on travel will not be lifted to all countries just like that.
It will no doubt be Country by Country and is going to be hard time for all of us to plan anything.
As much as I love the 757 she is very much on borrowed time with TUI - Procurement of RB211 engine spares is now proving an issue too from RR due 787 troubles.