Just because they can doesn't mean they would. Clearly the retna scans are an immigration tool used upon arrival. The UK scans people entering the country.
With that being the case, South American countries could also implement this procedure, but Colombia (was it?) hadn't. Money could be a limiting factor or simply a different view on immigration policy.
Yes. If I really want to disappear someone and hide them from The Eyes I would strongly consider some remote location where maybe even internet or cell service is difficult. Off the grid. Make it so that the person is too far out of Gilead's reach for them to care about. Of course, might as well ship her to South America. I think the biggest difference would be the difficulty surrounding immigration. Keeping Daisy or anyone they want to hide in Canada eliminates the need for fake passports, arranging transport out of the country, and immigration restrictions/scans/etc...
Meh, maybe it would be just as easy to hide such an individual in Markham. Maybe Montreal. How many in The Eye speak French?