Re: Conflicts with existing local subnet
The first option is to re-subnet either local or remote LAN to avoice the conflict, which is probably not an option.
The second option, which would be the best but might not be the easiest, is to ask the 3rd party on the other end to SNAT their overlapping source IPs/subnet. Otherwise, routing problem happens on the local end when you try routing into the tunnel while the destination exist locally. You don't need NAT on the local side since the remote end doesn't need to reach the destinations that are overlapping.
Although the above second option should be relatively easy to be implemented with any FWs, if it's absolutely not an option for political, financial, or whatever the reason is the second option is to separate DMZ into a vdom and set the tunnel from the DMZ vdom. Then you have to set up SNAT on the local lan vdom to avoid the routing conflict when DMZ needs to route to both tunnel destinations and the vdom-link to the local lan destinations.