Are you going to take the tissue sample to your vet for a definite diagnosis? My heart is just breaking for you and Missy. I know so well (too well) what you and your 14-year-old companion are going through. It's the feeling of helplessness that's the worst.
Please keep us informed. I hope that sneezing out that blood clot will help her to breathe a little easier. Saying prayers for her and you...