I know that the part of Protector which needs to be with the site can't be somewhere else, but for the part which is outside the website, can it be share between the different sites ?
Of course, YES.
XOOPS_TRUST_PATH can be shared by a lot of XOOPS sites.
Just put "one copy" in the host.
This folder contains files for bad_ip.
These files are distinguished by site's hash each other.
thank you for this information.
That's good to know it
Of course, yes.
This is the important spec of D3 modules.
(Though altsys and protector is not duplicatable...)