You can take photographs from the lock screen Robin. You should never hand your phone to somebody when unlocked unless you really trust them!
Defence in depth is a neat concept but, just like the person taking a photograph on your unlocked phone, you have to trust AxCrypt not to store your password. The point is that if you have “very special files” then you should not be storing these on your phone because there are too many variables out of AxCrypt’s hands which can lead to full compromise of your data.
I know you say you’re using SpiderOak and that’s a good solution but last time I tried it I was unable to edit files. There’s also Tresorit which is more expensive (true zero-knowledge) but by far more suited to mobile devices and desktops as it allows real-time editing, viewing files, locking with TouchID or a PIN, remote revocation, 2SV, limiting access by device, limiting access by IP range. It’s designed for professionals so may not be suited.
AxCrypt is geared towards users who store data in Dropbox, Google Drive, OneDrive (i.e. none of these use zero-knowledge encryption).