First off welcome to the world of boxing .

1. Yes
2. No/Yes Kinda vague but you can have all 5 accounts on one Battlenet account, but some like to split them to log in easier with authenticators.
3. Yes it would handle it 5 boxing Dalaran would be choppy, upgrading your Ram would help alot. 8 to 12 ideally.
4. Adding multiple monitors generally slows down performance but Im not sure someone else might be able to answer that one better than me.
5. Isboxer is plenty in terms of software, Jamba isnt a must but helps out immensly, as well as a ton of other addons. Search for Maxions setup and view his addons.