Thank you to Jorn @ Webflow for giving me some options. I will post his recommendations here just incase anyone else needs them:
About the first issue on desktop, it’s actually expected behaviour but I understand you don’t want it that way. If you select the the top slider element - movie-slider - and then head over to the slider settings start to go through the slides with the arrows buttons there you can see that it does go through the slides in the way it’s set up. The last slide is the one selected last but that will then leave the a space because the mask is set to 25%.
I’m actually not sure how to build what you want with the built in slider component.
I would probably look elsewhere because that component has very basic functionality.
Here’s a third party tool I recently discovered that might be something you can use in your project: How do you create custom sliders in Webflow?
Note that the Webflow support are not able to support how to use third party tools and applications.
The second issue on mobile was just a human error where some nested classes were getting hidden on mobile by accident.