I bit the bullet and shelled out the money for the Henjar Photo replacement plate for the 383 gimbal head. It is worth it. It provides a stable base to attach my long lenses to.

It is a common issue with Sigma's Zen coating. The best way to deal with it is to remove the coating. Rubbing alcohol and a clean cotton cloth will do it. Be sure to wear gloves to keep it off your hands. I did this on a 500 f4 lens a few years ago and it is fine.

I would go with a walking stick/mono pod combination if I were you. Some churches or shrines don't allow mono pods, so keep that in mind. What I use in those instances is a good old fashioned string mono pod. Stick it in your pocket and it's always available. Google it.

It's a little out of the way, but National Camera or West Photo in Minneaolis are the last and best in the cities.

I did this trip a couple of years ago for the same reason. The area around the Pinnacles is a great spot, as there are places to safely pull off the road and park. Look for any spot that gives you a view of the western sky and that you can have something of interest in the foreground. If you are going to try for the northern lights as well, stick closer to the eastern end of the Badlands loops, as the lights form Wall give off quite a glow.

In theory....

Have you thought about using some sort of cart to carry the tripod with? I have a friend who uses an old golf bag/hand cart. It does wonders with a heavy tripod. He looks like a fool, but it saves his back and knees. He can go down any trail that he can hike or walk on.

In the real world, when that cute baby bear appears, you may not have time to do much more than grab your camera and squeeze of a few quick shots. So if traveling where such wildlife interactions are possible, keep the longer zoom on and be prepared to crop later if need be. (Landscape doesn't move much, so you will have time to change to your shorter zoom if need be.) I wouldn't get too hung up about pixel peeping for those unexpected encounters. You are better off grabbing the best shot you can get with what you have at the ready. You may never have that opportunity again. A technically "imperfect" shot in that situation is still better than not getting it at all.

Actually, the 150-600 V2 does come in A mount. It's one of my favorite travel wildlife lenses. The tap in console is worth taking the time to use, as it allows for focus adjustments throughout the zoom range. I have it set for 150, 300, 450 and 600.