I've checked the route from Kirkby Stephen to Blackburn.
The climb from Garsdale Head is a killer! Fair enough if that's what you want to do but check those contours! There is another tough climb back up from Dentdale to the B6255 above Ribblehead.
You have definitely got at least one section of bridleway in your route! The section between Little Mearley Hall and Pendleton Hall near Clitheroe isn't a proper road. If you get as far as Pendleton Hall, the lane from there to Wiswell and over the A59 takes you to the old Clitheroe Road to Whalley which is a much nicer way of getting there (we used it on a forum ride last month).
Unless there is a particular reason to go via Downham, however, I'd recommend that you ride from Grindleton through West Bradford to Waddington village. The Country Kitchen cafe there is very cycling friendly. We use it a lot on our forum rides. From there, ride to Whalley via Bashall Town, continue towards Longridge Fell, then turn left for (Great) Mitton and continue on the B6246 to Whalley.
I don't know whether you
really want to be going up Whalley Banks! It is reputed to be an extremely tough climb meriting chevrons on the OS map. The climb below it up Whalley Old Road would be much much kinder on your legs on such a long ride!
Good luck - it sounds like a fun ride!
As for luggage carrying - bodge a rack on like this?