Actually... these are Disney constraints. These are additional conditions that exist for the special European tickets. These tickets are not sold to US customers, and hearing the US ticketing offices give incorrect info is not unusual.
Look at Disney's own UK ticket offer..
http://www.disneyholidays.co.uk/walt-disney-world/tickets/
Down in the fine print...
"2016 tickets are for arrivals between January 1, 2016, and December 31, 2016 and must be used for the first time at any park listed above by December 31, 2016."
You will find the similar constraint with other UK ticket sites selling the ticket... such as on this site
http://www.attraction-tickets-direc...y-world-resort/disneys-14-day-ultimate-ticket
Orlando Attraction Tickets (a UK ticket seller) is less clear on this constraint, but is listed on their site.. They say "Tickets are valid for the year selected"
https://www.orlandoattractiontickets.co.uk/Ticket/Walt-Disney-World-Resort--14-Day-Ultimate-Ticket
But their T&C page does outline they can't make changes to tickets once linked to MDE.. and they can't cancel tickets that have been sent out..
"
Disney Cancellations
Please note that we cannot accept any cancellation of Disney tickets that have been dispatched."
I'm afraid here you've bought into a system that is constrained by Disney (due to the special nature of these UK tickets) as well as a ticket reseller that is limited.
The Disney agent does have cancellation options with penalties after a ticket has been sent. I doubt the ticket broker is going to help you much, nor is a front-line Disney CSR. Your best bet probably is to get a contact address from Disney, and send them a letter and documentation asking that your tickets be exchanged for a 2017 ticket, or other compensation given the extreme medical circumstances.
You're pretty much locked in based on what you bought, but you can still try to find an exception process from Disney hopefully. I don't know if the UK people would be more helpful, or if it has to come from the bigger corporate entity. The fact they are international only tickets complicates things.