I'm an award winning filmmaker currently living in Long Beach, CA.
ND is what you need. Variable is a good choice, but be careful with color casting from cheap glass.
You might also want to look into something to diffuse the light to get rid of harsh shadows. The pros will use a big silk, but really I'm sure a big white sheet could help almost as well. As long as it's somewhat a thin sheet.
I would go with the A7s. Then, for bigger more professional shoot that require it, rent a cinema camera.
Waiting a few weeks is probably in your best interest.
However, at least once a week lately, something comes up about the Samsung NX1 that really has me considering that as my next camera.
I have the A7s and Commlite adapter. It works perfectly fine with my nifty 50 canon and my old T2i kit lens, which is APS-C crop. In order to use that lens properly, I either have to zoom in to avoid vignetting or switch the camera into APS-C mode. I haven't used the Tokina or the 24-105 you mentioned, but I imagine you should be just fine.
I hope this helps.
I believe the answer is twelve.
Just kidding. Your question is very subjective and an accurate answer would require a lot more insight to your short film intends to say.
I would suggest going and watching some of the films that are inspiring yours and get a sense for how different your main character is at the beginning from the end. And after you've gathered that info, try and squeeze that information into your character for your film. If you can't fit it into your desired 20 minutes, go through your character's changes and try to figure out how he can get from Point A to Point B faster.
The main thing you'll want to consider is that you don't want to have too much information thrown into your film. Especially if it's a short film. It's best to keep it simple. The more simple your character is, the less exposition you'll need in order to tell your story.
I recently switched from a Canon 6D to the A7s. I bought a Commlite adapter for around $100 and I haven't had any issues with any of my lenses. This, however, is not a speedbooster - just an electronic adapter so you can still use Auto Focus with your lenses.
Because the camera is full frame, the most you'd be doing is giving yourself a vignette if you add it for use in full frame mode. In Philip Bloom's review, he added the metabones speedbooster for use only in APS-C crop mode. The helped reduce the rolling shutter issue that you'll experience in full frame mode while almost giving you the same field of view that you get in full frame mode. However, unless you plan on only shooting in crop mode, you'd still need another adapter to use for full frame shooting.