@Teaceratops Ignoring time and cost issues, my vote would be this 'cruise to Vancouver, trans-Canadian rail tour with a break in Montreal, cruise from Halifax to the UK' option. Seems easiest visa-wise and has the benefit (?) of getting to stop in with yous along the way.
This whole situation sounds really complicated and stressful, but hopefully will make for an amazing family story once this is all over??
I looked up a cruise although I imagine you have already done all this sort of thing: https://ozcruising.com.au/searchcruise/bysearchbar/0/-111/-111/639/false/-111/-111/-111/-111/all/693