Wow!
After so many years together I can imagine that the fears and uncertantites your facing right now, both the tandeming prospects and the "chronic small vessel syndrome", must be very frightening and depressing. I too hope you see a healthy and favorable outcome to this setback.
In regards to the upcoming tour though I also can't help but think on your great good fortune. If indeed you can not tandem through the tour, how wonderful it is that you both have alternate bikes that you can ride instead. Granted, you'll be on separate bikes and loose some degree of closeness and it would be a break from your long standing tradition...but you definately do have the proper bikes with which to ride the tour together.
My thoughts are....Count your blessings my good man, and we'll all pray for a favorable outcome for your wife.