You cannot be left stranded by the airline, if you opt on your own will to forfeit your seat for a refund or credit of some description, your contract with the airline ends.
There are very clear rules laid out in those circumstances (EU261) which along with general 'duty of care' related stuff (LRV's and such like) they also have to get you onto the next available flight or arrange alternative transport for you. Your contract with the airline is that they will get you from point A, to point B, somehow. Any hint that Ryanair is exempt from these rules is completely false, whether you pay a penny, a £ or a hundred £, they are obliged to stick to the rules.
Airlines in general hate EU261, and you will normally need to request the literature stating your rights from a desk, bottom line is they're breaking the law if they refuse to honour anything.
So yeah, to conclude, book your seat in confidence with Ryanair, leaving you high and dry isn't an option.