these questions are not really about GL devices but about Linux in general, they are basic questions about samba.
I will try to answer them anyway.
As long as they can reach your storage - yes, of course. So make sure that you don't make it reachable via WAN without VPN for example.
Mostly no. This depends on the samba version (and SMB protocol version). To answer this question, you need to share which GL device and which client you will use. But in general, the answer is mostly no.
It's up to you to encrypt files by yourself. There is no plugin and no crypt-storage. Cryptomator should work fine.
It's accessible for everyone by default, as long as they have the credentials. You might set additional rules - but this is an advanced desire and needs manually editing config files.
All in all: The file sharing function is pretty basic and more like an USB stick in your router. It does not replace an NAS.
A NAS is more advanced, has more power and more features - like encrypting storage for example.
All is possible with OpenWrt as well, but not outside the box and requires advanced skills. A NAS is easier to use with these features.