Most of my mileage is two up on the Bird. Carol loves it.
Re screens.
I've alternated between the original and a pyramid plastics double bubble. For sub 100mph there is nothing wrong with the original for me. The double bubble makes it a bit easier for high speeds especially if you use a tank bag because you can't "get down" on the tank.
Re seats.
I prefer the original and have "done" France, Germany, Czech rep. and Denmark with no probs. Carol much prefers the Melboy bladerunner seat we have. It has gel inserts which I am not sure make any difference. The main aid is that the seat is more contoured/shaped, which stops Carol sliding about.
You can buy anti slip covers for the pillion which I think are effective. Lensman (Paul & Polly) use one of these.
Having been down the route of full hard luggage, we now travel with a lot less gear. Sports rack with holdall, Baglux tank bag and small rucksack. No probs.
We are all different and what works for one person will not work for somebody else, so it is very difficult to recommend things. I can only tell you of our experiences.
Hope all goes well and you have many happy miles touring.
:beer: