I want to know, if appeals on plagiarism are made, do users opinions on the matter actually count or we are just witnesses to see how the affected will be able to convince the admin(s) that banned him.
It's hard to say. The user sometimes has to prove why he should be unbanned, how he is a positive member of the forum, and what the community would lose if he is no longer here. It obviously helps if other members show support for that individual.
My answers for your questions below are not related to this case, but how it is in general.
1. Was the user accused of any other plagiarism save the one in OP?
You could get banned for just one instance of plagiarism. But usually there are more unless a mistake was made as was the case with Mpamaegbu.
2. Was the intention behind the plagiarism to pass the content as his or to make money?
If the member is in a signature campaign at the moment the post was plagiarized, the punishments tend to be more severe. It's either a permanent ban or a 1-2 year signature ban.
3. If mods or self locks a thread, can the thread creator edit the contents of the thread even when it's locked?
If you lock your own thread, you can open it again, post in it, or edit your posts. If the mods lock your thread, you can't open it yourself or edit existing posts.
4. If someone is accused of plagiarism in a particular post, does the person have the right to edit the post(add citation)under accusation?
Accused or not, you can always edit any of your posts or delete them. But usually if a copy-pasted post is discovered and reported, mods will delete it and it can already be to late for you.
5. Has the user contributed meaningful to the forum over the time?
That's the most important factor to determine whether he will be pardoned or not. That and the share amount of plagiarized posts in question.
6. Are some plagiarism cases given accelerated attention than others?
Some users can be unbanned the same day. But there have also been cases where users have waited a year or two for a second chance.
7. Is there some considerations in treating a perpetual plagarist and a one time plagarist?
The more posts you plagiarize the harder it will be for you to not get banned.
8. Can any other punishment other than permanent ban be given a one time apologetic plagarist?
The post in question gets deleted and no other actions are taken. Admins also issue 1 and 2-year signature bans.