Re: sensor. You can clean it yourself if you're concerned about it... but most of the time, a few blasts with a Rocket Blaster will take care of it. There are a few horror stories around here of people paying a lot of money for professional sensor cleaning and it comes back dirtier than before!
Make sure there that sensor is actually dirty before bothering doing anything about it... usually that means to shoot something plain white (like just a piece of paper) at a very small aperture (like F22) then do an "auto levels" or similar when it's on your computer. (The free Irfanview can do this if you don't have a fancy-shmancy image editor.)
Not necessary; this only affects the camera getting an exposure reading. Since you are setting your own exposures by using bulb mode, this will not make any difference. There is no worry of light bleeding through to your actual picture either, since the viewfinder is blocked off when the photo is being taken.
Ok, so with all that I haven't decided where I want to stand, how I want to beat senseless the late comers that will stand in my way and which one of my lenses to use. (Tokina 11-16 F2.8 IF DX, Nikon AFS Nikkor VR 18-55mm 1:3.5-5.6 G, or the, not really for fireworks, Nikon DX AFS Nikkor 55-200mm 1.4-5.6 G ED).
Generally, most lenses will give you a nice sharp photo, but one with more aperture blades (generally that means higher-quality lenses) will produce more points on the stars you get around brightly-colored static lights in your shot. (You'll see this more in non-fireworks shots but you still still see it happen sometimes in fireworks shots especially if there is a shell still flying up in your shot.) Most cheap lenses have 6 aperture blades, which means a 6-pointed star. Better lenses usually have 8-9. An odd number will produce double the points, so a 9-blade aperture will actually give you 18 points.
For night shots using ambient lighting that are not fireworks related what settings do y'all recommend? I'll still be using that tripod and the remote release. Should I trust my meter on automatic mode for shutter, F stop and ISO and bracket around that + - EV steps (2 each way)? I would like to do HDR post processing as well, that's why I would bracket.
Since you are on a tripod, it's definitely worth bracketing since there's no downside to doing it. Take a look at the shots afterwards to make sure that at least one of them seems to have the exposure that you like. If you're after HDR, make sure that between the shots that you're taking cover capturing all the shadows and highlights.
This sunset/night photography has got me flummoxed as to whether or not I should depend totally on my auto program or go with some combination, say Shutter priority, or the Night Portrait mode (i.e. slow sync flash, Flash for subject in foreground, shutter then stays open to expose background). So what do you think??? (Tell me Tell me!!!)
I always do Aperture priority and if necessary, use exposure compensation or change the metering mode.
Overall - you're got a lot of good info there but I think you may be overthinking it a little bit.

As long as you are shooting at your lowest ISO, you are not likely to need to do noise reduction on the final image. Manually combining extra shots afterwards is extra work that you'll probably not want to do, at least not for a while. If you're going that far, you can often bring up or down the exposure on just the castle without needing an extra photo. (You
are shooting in Raw, right?)
The thing I've found is that it's easy to take a
lot of photos during a fireworks show. I tend to figure on a few ones that are "off" and even without trying very hard, get more than enough that are "good." To the point where I was happy enough with my Wishes shots from my first DSLR trip (I shot it three times, using an entry-level DSLR and cheap lens twice and once with a fisheye) that I really haven't bothered shooting it since then. Illuminations is another story though

it's much more challenging than Wishes IMHO and there are many different ways of shooting it. I guess what I'm saying is to not stress out too much - as long as you have the basics (tripod, remote shutter release, low ISO, good aperture), you'll almost certainly get some shots that you're happy with! Best of luck!