I posted these a month ago but received no response.
It's not possible to use Custom Characters on Android due to the files being expected in a place that can't be accessed via File Managers (presumably the internal app data folder), it either needs to be set to an accessible directory, or allow the user to select one.
On recent versions of Android (13+ I assume), apps cannot view files not created by them without being given filesystem access, which prevents the user from being able to import saves under normal circumstances. It can be worked around by exporting the save on Android, and copy/pasting the contents of the save you wish to import into the Android save, but this isn't something that everyone's going to be aware of.