If I buy Unity Pro can I submit the apps to iOS and Android, or do I need to also buy the iOS Pro and Android Pro? And also if I take the subscription option are the apps that were already submitted going to show a unity screen if the subscription expires?
You need Android and iOS pro if you will use pro features when exporting, for example render textures, and you want to remove splash screen. Otherwise, if you only buy desktop pro, then you can only do that for desktop.
Unity pro desktop and android free will result in the free android version disabling any pro features (it should still work).
Subscription is editor based and export based, it won’t suddenly alter the status of any published games. However unity has a 12 month minimum period you can subscribe.