You have a lot going on in your post.

I would recommend against trying for high frame rate film acquisition for anything other than a tech demo. While the concept is sound you will be limited to short takes without having custom mags created. Additionally you need to take into account the ability for the camera bodies to get up to speed and wind down will be longer than for traditional 24fps. Factor in there are no new camera bodies, so you will be over-cranking at the newest a 20 year old camera.

Now if that doesn't scare you off and you go digital the cost and complexity of the project go down a lot. The main thing you need to keep in mind is how do you expect people to see this movie?

Peter Jackson literally had the influence to have special projectors and DCP players made and installed to show his film. Many top end post facilities will be able to master you a DCP, but very few theaters will be able to play it.

You can however attempt to master for a 60FPS youtube presentation as they are an available distribution format. The problem with that is getting paid. Putting a big expense into a production budget that can only been seen in a limited revenue distribution channel will force you to make compromises elsewhere.

If you happen to have access to a distribution channel than can show HDR then you can ignore all of that.

Technically there is a separation between acquisition temporal resolution and display temporal resolution. There isn't the direct link you are making between camera frame rate and display refresh rate. Different aspect of the camera, such as shutter angle and display type keep the delineation.

I am not sure where you got the 55 refresh number for the eye. Perhaps you are referring to flicker perception? Even then that is brightness dependent. There is a pretty marked difference in watching 120fps vs 60fps in terms of image motion appearance.
Doug Trumball gave some presentation of material at differing frame rates using modified playback and projectors to show the material (http://cinefex.com/blog/ufotog/)

Try to focus on the effect of HFR and how the framerate 'unreality' can be used to further a story. There are really interesting effects when you have characters interacting at different framerates.

Imagine a shot captured at 120 fps. You then use motion flow to make your 'super person' the 60fps max. he moves in an unnaturally smooth fashion compared to the other characters that are motion flowed down to 24 then back to 60 so 120->24->60
It makes most shots VFX shots, but it is a really cool effect to see. 2 people moving at the same speed and interacting with objects that react the same but one is different.

Cheers

One of the big differences you may be seeing is the level of acceptable support from pro-level equipment vs prosumer level support.

A 3 day turnaround would be excellent for a prosumer product, with basically anything under a week being good.

A 3 day turn around for camera support would be very unacceptable in a 'Pro' setting. If you are paying for locations, sets, crew and talent, each hour of a technical problem can eclipse the cost of a camera body. The pro-level cameras, from Canon; ARRI; RED; and Sony, you can pretty much get a direct connection to Pro-support. If you let them know your shooting schedule in advance, you can get same day service with direct response to your calls.

Depending on your expectations, the same exact service and the same exact response time will get a very different recommendation.

The a7sII is a camera that can make some great images and video. It can also pass for a 'normal camera' in places you don't have a permit for.

What is your target deliverable? Is it for Youtube, BluRay, streaming or theatrical? For the most part you should be able to make great quality video.

One of the best things to look at for that is going to be the 'slog' functions. There are picture profiles for slog(1/2/3). They require more post color processing but give a more options for image look. That would be the first modification you can make to upgrade the quality of video.

The next would be to use an external recorder. I have really great things about the odyssey, but I have not felt the need to use it with mine.

As for trying other cameras, depending on where you live camera rental houses are usually very willing to show you their gear at their place. So you can try out a few different camera's and rigs if they aren't busy and you are nice about it. Bring a friend so you have a person to shoot footage of.

How is the power zoom? I haven't heard great things about it and went with the gmaster 24-70.

In terms of pure image quality you shouldn't gain much going from a A7SII to a Blackmagic Ursa camera. Do you have a specific reason that you are interested in the URSA? Have you rented one to try it out?

For the most part 4k shouldn't have much of an effect on product. In terms of images quality the difference between a 2k/HD image and a 4k image is a very subtle one until you get to extra large sized screens.

Steve Yedlin (A.S.C) has been going over starting to present on this ( an early article is here http://www.yedlin.net/BigK_2014.html) He has a demonstration of 2k all the way through (15 perf 65mm 13k) for 4k cinema finish. There is asmall image difference between the 2k and 4k and there is virtually no difference going above 4k(6k,8k,13k) for a normal narrative.

The best reason to go with a 4k on acquisition is because of a distribution requirement. But if you don't have that requirement, most projects will be better off to shoot on the highest quality 2k/HD camera that you can rent for the best looking projects.

When people talk about this they typically mean that they crop into a 4k image to make hd or 2k sized extracts. It is a technique that allows for a single camera to act as if it's a multi camera setup. So you an import footage as 4k into a HD/2k timeline and repo it to get the framing you want to have.

Although people talk like this is an easy thing to do, I have seen a few directors very disappointed in trying it out. For this to work all of your footage needs to be shot with this technique in mind. So you need to shoot with sharp lenses, flat light across any of the areas you may want to crop into, and a VERY deep depth of field.

It's a frustrating situation when an image looks tack sharp on an editorial monitor, but on a screen or large monitor appears soft, or worse yet half soft, because of focus issues.