Why does Password Safe complain when loading Bimil files?
While formats between Bimil and Password Safe are compatible on low level, that doesn't mean they are the same. Some fields types, while officially assigned to Bimil cannot be read and modified by Password Safe. These fields include credit card information and two-factor authentication. Other fields can be freely interchanged.
Why is extension not .psafe3 but .bimil?
As Bimil supports some extra fields as compared to Password Safe, it is safer to use its own extension rather than .psafe3. In a pinch you can open Bimil files in Password Safe but be aware some fields might be missing in action, i.e. credit card information and two-factor authentication.
Can I safely open Password Safe files with Bimil?
Yes. Bimil will not remove any fields you have in your .psafe3 file regardless of whether it supports them or not. However, that doesn't necessarily work both ways as you can do some things in Bimil you cannot do in Password Safe. Best examples for this are credit card and two-factor authentication fields. Additionally, Bimil allows multiple fields of same type while Password Safe does not.
Why bother with Password Safe compatibility in the first place?
Password Safe format has been designed by Bruce Schneier and it is generally thought as the best format for this purpose. While original Bimil format was a bit more flexible and, I believe, secure, I've decided to go for something that has been peer reviewed.
Cannot update package - APT says it's being "held back"
If upgrading from older version, it might be necessary to upgrade distribution files first. Try using
sudo apt-get dist-upgrade.
Is source available?
Yes, source code for both program and its Password Safe library is fully available on GitHub.