Well, there is a bug (assigned and scheduled for a fix) in Google Play Services that breaks being able to cast and use hotspot on the same device. You can use an old version of Play Services if you are rooted. Otherwise you need two devices.
Having said that, I have unlimited data, but I have found that you can use your hotspot so the Chromecast works, but then use Plex to stream locally synced content. This way you don't use your cell data plan.
I actually use a Chomecast Audio in my car since it's way better sounding than Bluetooth for music. So I am all for unconventional means
If you have no intention of storing movies on your phone, you could Plex sync the content to an Android tablet then stream from there. Or you can run a mini Plex server on a PI and cast it to your Chromecast in your car without using data. The only time the internet would be used is when the Chromecast wants to update it's firmware.
---------- Post added at 01:36 PM ---------- Previous post was at 01:32 PM ----------
As for Localcast, you can store the data on anything that is DLNA compliant machine/box in your car or SMB share (like a PI box), then just fling it to your Chromecast.