Hi Herman,
No experience with Minilock or with third-party encryption on cloud-based e-mail. My 2 cents - adding extra encryption to an already encrypted product doesn't buy you much more security. Here's why:
Each user on a Chromebook is allocated their own encrypted space for files. There isn't an explicit concept of a super-user such as an Administrator (in the Windows model) where that elevated account can gain access to all files on the machine. If you cannot login as that person, you effectively cannot see their files, even if you ripped out the drive and tried to dissect it using forensic tools.
If the attacker/hacker can access the files on your Chromebook, they can get to your files on Drive and GMail because it's the same username and password for all three. If they can't read your archive files on the Chromebook, then they can simply launch a browser and get to the online copies.
So the real security here is two-fold - 1. make sure you have a strong password and consider changing it on a regular basis if you suspect you've been compromised somewhere; 2. think about using Google Authenticator as a means of off-setting any unauthorized changes to your account.
Hope this helps.