I'm going to take a guess here and assume the plan is to wait 5 days. The CDC is currently saying if you feel better and don't have a fever 5 days after testing positive, you don't have to isolate any more as long as you wear a mask like all the time, and try really hard to stay away from people. They also say to not travel for 10 days. but that's just a recommendation. In theory, if Tomomi has a nice, light case, they could probably do the Seattle show. Of course if anyone else tests positive between now and then, it's off.
And when wearing a mask, I'm not talking about a cute one. I'm talking about a big ugly N95 mask that makes you look like a demented bird. I myself am taking delivery of some highly fashionable black N95 masks this very day.
While I'm VERY concerned for Tomomi's well being, I'm also hopeful she will recover without too much discomfort or difficulty. I'm fairly confidant that will be the case. If the other members by some miracle escape positive test results, the show will eventually go on.
All in all, I'm happy I kept my London ticket all this time. I also will probably just eat this tote bag of Japanese snacks I got as a gift for the Vancouver show, in case they were starting to get tired of American food.