1) No, due to a problem Netbalancer and QoS don’t work together. If you bond the vpns there is no point in netbalancing them too.
2) Maybe they are trying to access the dns server of one ISP via the link of another. Maybe it is because QoS is not working along with Netbalancer, so you suffer from downloaders.
3) Yes you do assign an IP on the BOND and use it as default gateway for the internet. It could be either public or private, usually there is no need for public IP.
4) You have to be aware that the slower link (on the upstream) will slow down the other 2, as it will not be able to send packets with the same rate as the other 2.