Wouldn't heat become an alternative worry if you do that? The motor is air-cooled, if you stop airflow then it might stay dry but then overheat instead?
I cannot begin to imagine the load that 2 emtbs plus this sail will put on your rack. It'll be many, many times over the load threshold for the rack and towbar at highway speeds, and I'd guess you'd be lucky to get even half your usual fuel economy.
I've seen those. Seems like it would put a truly enormous load on both the bike and carrier? It's effectively a couple of 2m^2 sails hung on the back of the car. Do you have any concerns about the bikes being ripped off the rack? I had a massive gust of wind rip a correctly mounted roof rack off...
My Golf R Estate is the same. No tow rating. I found a fitter who was happy to put in the bar and electrics, they all fit perfectly, but I have clear instructions from TowTrust that while a bike rack is fine, towing would be illegal.
Your issue is likely to be the maximum weight the tow bar can take. On a disco I think it's 66 kg, meaning you're going to be 10-15 kg over once everything is fitted. Probably no problem, unless there's a problem...
I had years being spoilt by an Accord Tourer which would easily take a bike whole and loads of gear. You could even take both wheels off and get two bikes and a lot of gear in the boot with the cover over. Brilliant.
I'm now running a Golf R Estate. Both wheels on is a no-go, but it's easy to...
I've been using an Atera Strada M3 for about three years. The mount onto the towball is super easy and I've never had it move. It's so quick that I'll take the rack off and throw it in the boot when I park up for a ride.
I bought the longer wheel straps for two slots as I used to have a plus...