Hi Palak,
How are you?
This issue is resolved. It turns out the preference stated in Cordova document and apple developer document is different. There is a missing "s" after "allow" for Cordova.
Apple: allowsInlineMediaPlayback
Reference: https://developer.apple.com/documentation/webkit/wkwebviewconfiguration/1614793-allowsinlinemediaplayback
Cordova: AllowInlineMediaPlayback
Reference: https://cordova.apache.org/docs/en/latest/config_ref/index.html
I put both of them in Extensibility Configurations and set to true. It works perfectly after generated a new version of app.

The file is about 150mb. Things I concern a bit is the usage of mobile data in case of device without Wifi.
For the second and third questions, I haven't try other ways but my back up plan is same as yours which is using GIFs. I would probably reference this post if I use GIFs.
Hope the information above is helpful :)