The following is a great site that has a lot of good information on it:
http://www.dr-addie.com/
I recently acquired a kitten, that was ill, and the vet told me we had to watch for FIP. So, I did a lot of studying up about the disease, and it is not an easy disease to really pass to other cats. I have a friend who does a LOT of cat rescue work, and she has her own rescue group. She has lost cats to FIP and she assured me that I should not get upset, because I have other cats.
It turned out, so far, that he does not have FIP, and since there is no definitive test for it, except a necropsy, after the cat has died, we are just praying that he does not have the dry form of FIP. If they have the effusive FIP, yes, the belly swells up, and x-rays will show fluid outside the lungs or in the abdominal area. He showed no fluid, and he seems to have made it over the hump of being so ill.
Your first question, about the virus mutating inside the particular animal is usually what vets believe happens.
I definitely would not vaccinate another kitten, but if it has been a while since your kitten died, (although it sounds like it is recent?) I would wait on getting or bringing another cat into the house, until some time goes by. This disease is very strange, and even though it is not as contagious as once thought, I wouldn't want to take the chance.
With the two options you mentioned, taking another of the ragdolls, or saving a stray, I would opt to save a stray. There are so many cats that are in need of homes, in these shelters, and in places like Petsmart or Petco, where some cats are desperate to belong to someone special. I was just in Petsmart today, and saw a six year old gorgeous cat that is there, because either the owner died, or someone just didn't want the poor cat any longer. A beautiful calico female.
I believe the site above, though, will help you understand the disease better, and give you reliable information about the disease. I do not think that dogs carry the disease. But, you can double check on this site that I gave to you.
Also, there is a Yahoo group that is great, that has a lot of information and people that have been through what you have been through, and you can join that if you want to talk to others about this disease.
The address is
FIP-subscribe@yahoogroups.com
God bless and I hope that you will be able to make a choice that is both well informed and one that will give another cat a chance to be with someone who sounds like they have a great heart, such as yourself.
Elena Marie
"Open your heart to learn all you can in this life, and to give to others, the joy and satisfaction that they may remember you as a true friend."