In the case of Evilnat's CFW, you can look at xai_plugin as its features. It's a file located in dev_flash/vsh/module that is responsible for all the
cool stuff the CFW can do. Without any disrespect intended, Evilnat CFW without this plugin is essentially just Ferrox.
To answer your other questions:
1. It's not crucial to update, but would be recommended to download should there be one available. When an update is available, you can use the option in Custom Firmware Tools.
2. There are currently no updates for 4.90.
3. Other than the overclocked, nobt/nobd patched versions, I don't there will be any other versions of 4.91 (excluding any potential future updates).
EDIT: if a moderator can fix my link, that'd be appreciated.