I think this is right but to make it work you'd need to do one of two things to pull it off. First off, if you're doing it just for Web the nginx proxy putting original ip in the header and unpacking on the other side is the smart move. Otherwise.
1: route all your traffic on your side via the vpn, and have the routing on the vpn side forward the packets to the intranet ip on your side not do dnat on it.
2: if you want to route normal traffic over your normal link then you could do it with source routing on the router. You would need two subnets, one for your normal Internet and one for the vpn traffic. Setup source routing to route packets with the vpn ip addresses go via vpn and the rest nat the normal way then the same as before, vpn on cloud forwards not nat to your side of the vpn.
In both cases snat should be done on the cloud side.
It's a fiddly setup just to get the ip addresses though.