As the letter is dated 6th Dec you have until 6th Jan to request the MR without any argument. It might be possible to request it after that date if you can show 'good cause' for the delay. What constitutes good cause is not laid down anywhere but failure to receive the letter should be accepted - as long as DWP accept that it didn't arrive.
In your situation, given all the problems you have had thus far, I would get the MR request submitted in good time!
The planning a journey etc will only look at mental health, sensory and cognitive problems. They will not consider worry about your physical health causing a problem unless this is part of a general 'anxiety' diagnosis.
Nothing on this board constitutes legal advice - always consult a professional about specific problems
