If you haven't already, learn how to manipulate the camera in the dark, including zoom and focus. Don't expect AF to be available. This also includes putting the camera on or off the tripod and adjusting that.
Put opaque gaffer tape over any external lights on your camera.
Wear sturdy boots, long pants and take bug spray.
Plan your shoot in the daylight. You can use apps like photopils to help you.
You will need a wider lens than that 105/macro, unless star trails are what you are going for.

Even though it probably can be done, I don't think I would trust it without checking before wiping the card for a new set of photos. That would require an additional device. That smartphone idea is intriguing.

I use that Tokina lens for night photography, mainly Milky Way shots. There are a lot of articles out there that talk about great night photography lenses, and usually this one appears on the list. Tokina also makes a newer 11-20 2.8 that you might check into as well, although I have no experience with it..

If you can, you should cut down your interval. 2/3rds of the night your shutter will be closed.

I use a sling bag on a motorcycle. Hike with it too. Never had the problem JD750 described. All of the bags of this type I have used (3) sit like a backpack, except the strap goes across the body. You can swing it around if needed, but it does not do it on own.

Noticed that there will be some sort of sale on Amazon for prime day of a Datacolor Spyder5PRO. Check it out if you are looking for one. I don't have any more details because the sale has not started as of this post.

Based on what you have said, I would recommend the latest version 18-135 EF-s lens, and purchase the hood as well. It works to get her going to learn her new camera, and is a good all around lens. After that, any lens purchase decisions should be hers, based on what she wants to do. Who knows--maybe she would rather spend available money on a flash, or a tripod, or something else.

How about this approach:
1. Figure out what you want to shoot. Then ask if your current equipment will work. If the answer is no, then you will also discover the reason why the answer is no.
2. Do a little research on what lenses or equipment would solve the problem you discovered. You don't have to become an expert in this area, and there will likely be several options available.
3. One at a time, rent a lens or two (or three!) and use them to do your shoot.. Who knows, you might need a different body as well.
4. Return the equipment when the rental period ends.
5. Process your pictures however you usually do, and evaluate the results.
6 Rinse, repeat.
7. If you find yourself wanting to rent the same lens or equipment a third time, think about buying it. Of course cost is an important consideration at this point.

This process will work whether you are shooting Macro to Milky Way, Birds in Flight to Street Photography. You can experiment and not make costly mistakes.

I am going to bet that when you are in Auto mode all is (more or less) well, but in Av mode things are inconsistent. Check out pages 227-254 of the eos80d-im3-en.pdf instruction manual (NOT the basic instruction manual.) Especially page 228, bottom blue box, and page 236. It is hard to tell, but it sounds like this is your issue: in Av mode, the flash acts as a fill flash (by default). There are menu changes that can alter this behavior--described in the cited pages.

When using a flash in general, aperture controls flash exposure, and shutter speed controls ambient light exposure. What is probably happening to you is that in Av mode, you have already set the aperture directly, but the camera meter is setting the SS so that the ambient light properly exposes the scene. This will result in longer shutter openings than you might expect if what you are interested in is mainly being illuminated by the flash.

Also this article may have more information for you:

http://cpn.canon-europe.com/content/education/infobank/flash/fill_in_flash.do

Buy the lens. Rent a second body if you feel you need it

FWIW, the light meter provides information before you snap the shutter, the histogram does so after. No reason not to use both.

Starstax software allows you to use jpegs to create star trail photos.

Im sorry, but this post makes no sense to me. If your goal is to shoot with the aperture "wide open", why use Av mode? Use shutter priority, and crank up the SS until you are there. Or do it with ISO adjustments. Controlling SS directly is way more important for fast moving objects than making sure your aperture is wide open.

So as to not be off topic, I think this post is bad advice (but in fairness, not the WORST advice). Av mode for birds...sure it can be done, but it falls short of best practices IMO.

I tend to straighten very early, because I find any undesired tilt VERY distracting. I prefer to crop early for either aspect printing or content, although this doesn't always happen because of a change of mind/vision. Corners are important, and knowing where they are before a lot of PP makes a big difference in time and effort.

Calibration is also useful for multiple monitor setups, or if you ever use more than one computer.