4.1) Do I need ADD2PSU to connect those two PSU's?
http://www.add2psu.com/store/That would work very nicely, but (Not sure, check me on this one) the only purpose to using two power supplies is to use the extra pcie connectors to power the cards. That ADD2PSU thing is mainly used in server motherboards. The benefit of ADD2PSU is making sure your chained PSUs start simultaneously, or as close to it as possible. Otherwise, you may run into problems if some of your cards aren't getting power when the motherboard is POSTing, so they don't get detected.
Also, make sure the molex cable and PCI-E power cables for a card on a powered riser all comes from the same PSU. Again, this can be a source of trouble if you mix the sources.