It's one thing for a film company to be sustainable, it's another thing for film to be sustainable in the motion picture industry.

New labs can be good, but film is much more than just throwing stock through a lab.

There need to be new camera bodies manufactured. Film cameras are wonders of mechanical engineering, but being mechanical they are prone to wear. There needs to be enough camera bodies so your shoot begins when you want, not when a body is available. There also need to be extra bodies so that if there is an issue, a new body can be drop shipped to a set.

Right now as an industry, the best of the old bodies are being upkept and availability decides when you can shoot. If film actually takes off the constant stress of production on a dwindling number of mechanical devices will deplete the supply.

On top of that if enough productions shoot at the same time there need to be support experts that can go to set to troubleshoot issues.

New labs, new cameras, new scanners, new camera tech's and new films stocks are what is needed for film to truly stick around.

Sort of.
You have more resolution to see lens flaws in a still, but moving scenes will bring out flaws differently.

I don't think softer or sharper lenses are more or less appropriate for still vs motion.
It is all the look you are going for. You are comparing a good quality photo lens to a good quality cine prime. In broad terms both will faithfully render your scene.

If you are happy with your lens and currently shoot with it you should be ok.
If you are worried about things looking to sharp, use some diffusion to soften the look.

There are a lot of things to worry about , but from the shoot you described and the choices you have lenses aren't one of them.

BTW the Xeen primes should be sharper than a photo zoom lens.

You need to consider your entire audio chain and what you are shooting.

What are you shooting? Is it an interview or a live action piece?
Will it be in a controlled location or will you need to worry about wind and traffic?
How many people will you need to record at once?

Will you have a boom operator, a recording engineer?
How will you record, directly in camera, or into a separate recorder?
How will you monitor audio on set/location?

How will you post the audio?

What is your final deliverable? YouTube, BlueRay, or DCP?

To very directly answer your question.
No. A Xeen level cine prime will not improve the look of your shoot as you describe it.

As long as you aren't zooming in or out, or changing the lens settings during a shot you can expect the same quality of out the lens that you see in stills.

This depends on where you are selling your content.

Typically cameras that capture 4k vs UHD cost more, but are also targeted at professional motion pictures.

If your clients require that you can shoot on a native 4k camera, then you should look at the DCI(4k camera)

Otherwise there is no quality loss scaling up UHD to 4k.

Gregorio,
That was a good piece. There were a few standouts for both good and ill.
The parts where you had an actor still in frame, front on to the face looked great.

Where you can use some help is on the moving shots. You could benefit from some stabilization. A lot of shots looked handheld and that is creating a conflict with some of the speed effects you are using.

You can always add shake in post if you feel you need more for that handheld look. Having the shake in camera makes speed effects look 'jumpy'

I don't know that you need a gimbal, but a glide cam, steady cam or something similar can help out with the camera motion on moving shots and handheld.

If you can't work a tripod into your shoot, a monopod may help out, or even just a walking stick you can use to steady the camera.

Some kind of car mounted rig or gimbal will help for the driving shots. It can even be as simple as filling the handholds in a door with sand and using a post with a 1/4 20 screw on top to reduce camera movement.

Overall you did a lot with a little and you have great moments.