Only tighten the thru bolt when the wheels are on the ground. If you tighten it with the vehicle in the air and then lower it, you can deform the bushings.

Likewise, when you add grease via the zerk fittings, you are supposed to loosen off the thru bolt, add grease, and then tighten/torque the thru bolt, otherwise the bushings will bulge and you will no longer have a flat surface against the washers. This will lead to premature wear.
Grease will come out between the bushing and washer while you are tightening the bolt again.

a 19mm gear wrench really helps when loosening off the thru bolt for routine maintenance. Otherwise you will have to remove a bracket on the frame to fit a socket.

https://www.youtube.com/watch?v=xvMdAX2uybs

edit: to answer your questions:

1. yes, add grease before tightening the bolt. don't get carried away.
2. use a thin bit of grease on the surface of the bushing while you are greasing the sleeve. But grease will end up between the bushings and washers when you do number 1.
3. tighten with the vehicle on the ground