Oh no, sorry to hear that Laurie! Planning this one train journey has certainly been troublesome. I do like your idea of going early enough to catch the parade though, it is quite special to watch.
what would we do with our luggage while we wait for the cows? Is there
a place to store it?
Yes. There is luggage storage at the Lauterbrunnen train station. Also, the cows make quite a mess of the streets as they walk along - you will be happy not to be arriving right after the parade. Wait for the clean-up crew to do their work first before you drag your luggage through the streets. ;-)
does this mean I won't be seeing any cows on our hikes while we're
there?
The cows will be at their home farm, so in the Lauterbrunnen valley and also in Wengen, so you will still see them in the fields, but not at the higher elevations (Männlichen and such). In Wengen if you do the the Mönchblick viewpoint walk you should pass by cows in the fields...other areas as well, but I just know that one of the alpine descents goes right to a farm there.