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PfR shouldn't have any problem with multiple routers. The MC is what keeps track of egress pathes, whether on a single router with mutiple egress ports and/or multiple egress routers.
I haven't worked with Cisco's WaaS, so I don't know whether it shares state information between devices. This would be something to check.
"iWAN", eh? That's a nice marketing term.
I've (extensively) used VPN across Internet for a site-to-site connectivity. It can work fairly well (I've found it often on par with private WAN cloud technologies, e.g. frame-relay, ATM and MPLS), but if you want to manage application performance using QoS you really need to be able to manage all congestion points. Cloud egress, for multipoint, is a major issue. "Sharing" the Internet connection for VPN and "regular" Internet access is a problem too.