Yes.
Depends.
- You can try a custom TWRP build which attempts to decrypt Android 8/9 encryption and see whether you’re lucky and it works for you. It has been seen working here and there, but not reliably for everybody.
- TWRP backups are working, but if TWRP can’t access the data partition, then the data partition will not be part of a TWRP backup. Be aware that TWRP doesn’t backup the whole data partition anyway, even if it has access to it.
A backup of the files in an encrypted data partition can be made using other means while Android is running. ADB backup is a simple way, but restoring is a mess. Titanium Backup gets mentioned quite often for any scenario.
I wouldn’t count on it, as currently two experienced community developers tried to get TWRP to decrypt Android 8/9 encryption on the Fairphone 2 and don’t know why it doesn’t work (reliably at least). It seems quite elusive.