Access to members’ computers

(1) Let me try to explain the similarity between IP (Internet Protocol) Address and MAC (Machine Access Control / Media Access Control) Address:-
Every computer in this world connected to Internet must have a unique IP address and MAC address.
Now the difference is:-
MAC address is hardware dependent. This is inbuilt number of computer’s network adapter. Untill the network adapter is replaced physically, the number can not be changed.
Every computer connected to the internet must have a unique IP, its true. And they are assigned either static or dynamic way. A static IP is reserved for a particular computer. And no one, in this world, have the same IP address. To have a static IP, it costs Rs. 2000/- per year. It can be obtained from Reliance, BSNL etc. i.e. from Internet Service Providers (ISP).
In dynamic way, ISP assigns IP address to a computer, trying to connect to internet, a random value, which is unique at that moment. After few hours of internet usage, if that computer connects again to the internet, it may acquire a separate IP address.
Moreover, there is also a trick that lets multiple computers share a single IP address… and that trick is called a ROUTER.
(2) This forum is in phpBB platform (script) in which IP addresses of forum members, can be accessible to the forum Administrators and forum Moderators. However, this phpBB script is customized so that IP address can not be accessible to the forum Moderators. At present it is only visible to forum Administrator.
MAC address of the forum members can not be identified in phpBB. The phpBB developers have not yet implemented MAC detection script and possibly they are not interested in this.
So, MAC is not accessible to forum Administrator as well as Moderators.
(3) Admin has no access to member’s computer. As MAC detection is not possible, no question arises to record it. I have serious doubt about the ability of Admin regarding such matters.
(4) None.
(5) Sorry, I could not follow this point (5).
Hacking member’s account is possible; as such, use of a strong password prevent this chance.
Waiting for your reaction.

Many many thanks to nemo for his lucid explanations! I think it is now clear as to how the misconception and blind faith (existence of user names under same IP is indicative of duplication) can malign the genuineness of a member and convey a wrong message to others!