Do Bikers Have Passive Income

No - if you skim through https://googledrive.com/host/0B-Q7WnzcPvLtLVVoM3otTVlnOFE/all.htm you can see that basically anything with 4 seats is going to have a back window. My best experience with getaway vehicles so far have been the Jackal (inexpensive, easily stolen) or the Exemplar (200k or so, performs well.) Super cars don't appear to have back windows, or they're very small but I've managed to shoot and kill other players in them during a chase.

I'd suggest stealing a jackal and tweaking it out a bit to try, since you can get the car for 'free'.

Just to note, my experience with regular vans and cube vans is that the back doors or cargo door will stay open during travel, especially if people are shooting at pursuers.

Yes, it is possible to get a bounty while in passive mode.

Running you over is a way to kill you while in passive mode.
Another way to get killed by other players while in passive mode is getting hit by helicopter blades. The helicopter blades will also kill through walls, even when being in your own apartment if you get to close to the windows.

I recommend going into a store or your apartment and stay away from the windows if you are in passive mode and have a bounty on your head.