1. The newbies should be restricted to posting only in Beginners and Help Board.
Only this first suggestion would create big problems for the forum, as this board is just meant for beginners to ask some basic questions - but specific questions and discussions still need to take place in their assigned boards. I think that such a restriction would discourage many beginners, especially those who want to offer or seek a service. Even if your proposal were implemented, 90 days is simply too much to limit someone to just one board - and moderators are not the ones to decide who qualifies to be a member of the forum or share merits with them.
The Merit system is very well-thought-out and for me personally it has done a lot of positives for the forum, although there will always be those who will not approve of it for some of their own reasons.
2. The rate of plagiarism will reduce aswell.
Anyone who wants to plagiarize will do so anywhere on the forum, I don't think the B&H board would be an exception. The only thing that would make sense is to forgive beginners the first such offense - but that would again require some kind of record and a lot of work for the moderators who are not too many anyway.