I would think the tarp would create more wind resistance than just leaving the bike lashed to the roof. :-/
As far as the cats go, I'd worry about them getting under the pedals or having a freak-out in the vehicle.
I'd keep them caged and when you park to eat and such, let them out then. So long as the doors of the van are closed when you release them, there shouldn't be any trouble.
But I'll defer to the wisdom of those who have travelled more often...especially with critters. ;-D
I just worry because a) the carrier is *just* big enough for both of 'em to lay down - no bigger, and b) they can't use the cat box whenever they want. And there's no guarantee they'd use it when I stopped, so...well, I worry. I can deal with cleaning up messes, but I don't want them miserable. On the drive out, we let them out after we got well underway each day. Probably against the rules, but it worked.
On the way out here, they slept most of the way. And probably I'm worrying needlessly about their comfort. After all, they spend most of their time when I'm not home curled up in a little ball together on the bed...don't use much more space there than they would in the carrier.
Miss Kitty doesn't like car travel so I don't take her on long car trips but.....
The one time I let her roam the car, I had to fend her out of the pedal area. I won't do that again. :)
She does a lot better when I orient the cage so she can see me all the time.