It's up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you're okay.

Yes, but they are not accessible.  Although they are not visible or accessible via the class interface, they are inherited.

It is available to any sub-class (a class inheriting this class).

It is an actual offset (at the assembly language level) - not an offset into the IL instructions.

Security can be handled at the machine, domain, or sharepoint level.

Microsoft Windows SharePoint Services Technical security measures provide firewall protection,
intrusion detection, and web-publishing rules. The Microsoft operation center team tests and
deploys software updates in order to maintain the highest level of security and software reliability.
Software hot-fixes and service packs are tested and deployed based on their priority and level of
risk. Security related hot-fixes are rapidly deployed into the environment to address current
threats. A comprehensive software validation activity ensures software stability through regression
testing prior to deployment.