Not usually.
The WAN IP is still needed since it will be used for the firewall itself, just not for traffic leaving your LAN.
As for the outbound NAT rules, they are processed in a first-match-wins fashion. If you have three rules that specify traffic from LAN uses a VIP, it will use whichever one is on top, it won't skip it to use the next one down to do any kind of balancing.
If you want to use them all for your LAN, you'd have to specify the rule in such a way that it matched a different portion of your LAN for each VIP.