I wish them all the success and a great time, but what a mess!
The vehicle is well outside of its design limits, both weight and CoG, which could make it dangerous to drive and unreliable. Certainly, it'll suffer handling issues on poor traction surfaces, wnating to pirouette when turning, and will suffer off road from its overhang and extreme weight.
There is no reason to move an exhaust up there - if the standard SIII route is deemed too vulnerable, then simply re-routing the tail pipe through the rear wing panel would do the job - the SIII route is well protected by the chassis right up to where it pops out under the rear wing.
The front storage box is fine, as long as the mounting frame is strong enough, but the rear is a disaster - it appears to be bolted to the cross member, so most of the weight will be borne there, but the side members that wrap around the hard top are bolted to thin aluminium. Even with spreader plates inside, the bolt holes will want to tear open because the weight is overhanging the chassis mounting, trying to rotate the whole assembly aft. They may have added steel frames inside to prevent that, but that means even more weight. I can't imagine them not breaking either the rear springs or their chassis mountings.
What would have been better, if it's just the two of them, is travel lighter by packing less, fit out a Station Wagon with a roof rack and RTT to store kit more evenly and within the vehicle, or fit out a three-door hard top with a drawer and chest system in the rear load bay that allows a huge amount of safe storage and provides a full-width sleeping platform, with a roof rack if needed for extra lightweight equipment. Three door models are better for 2-person trips because their lack of a rear foot well and rear doors allow the fitment of twin front fuel tanks (though the shorter MoD ambulance front tanks may fit an SW) in addition to the rear tank, so no fuel cans are needed, and allows the installation of Jerrycan holders in the sides behind the front door for carrying 20l water cans or other fluids containers. That works extremely well in my 109, giving me up to 80l of water as well as 42Gal of fuel using standard (ie relatively easily replaced and safe, proven) parts and configuration.
I hope to do some extended trips with just my wife in my 109 when my kids are older, and for that will strip out the rear seats I fitted and add just a system of drawer and chests, similar to those from MobileStorageSolutions.co.uk , to extend the full length of the load floor to provide just such sleeping and internal storage. While the cost of doing so is high, a budget version could have been achieved by this couple with a few sheets of ply and some basic DIY fixings, much like some of the ingenious storage systems on this forum.
Good luck to them, though, for having the enthusiasm to get out there and explore.