Highly likely you've blown the rear SAM (Signal Acquisition Module). Even if you can acquire a used module, it will still need coding to the car so you'll need a good MB independant if you don't want to visit the main dealer.

The sensor for outside temperature is (probably) not exposed to sunshine, thus a reasonably accurate figure. The interior of the car is, essentially, a glasshouse with lots of radiated heat from body panels, etc. And exposure to the sun. So the climate control will have to work hard to keep the interior temperature at your required 20 degrees.

Why are there speedlimits anyway? I am allowed to drive at 60mph on a single carriageway road with no protection from the traffic traveling in the other direction, so 60mph in a contraflow is clearly safe as a driver. For the workers, any vehicle doing in excess of 30mph will kill them if they are hit, so any speedlimit in excess of that won't save anyone.... What Highways England need to do is organise their roadworks properly. Just because they are at some stage over the next 2 years going to be, say, putting in a section of so-called managed motorway over a stretch of 20+ miles does not mean that the entire stretch has to have a 50 limit on it for the entire period. You can only realistically work on about 2 miles at a time, so work on that, get it complete, then move on to the next 2 miles stretch!

A Mondeo will be wider across the rear seats than a Picasso or Scenic, so there is no drawback in terms of space, only in versatility. As the MPV's have individual seats, each can be folded or removed seperately, whereas the Mondeo has a standard 60/40 split. I can well understand the appeal of the higher seating portion, but given your very small budget and having your family in there, surely safety and reliability should be the priority. Your partner will get used to the car, and it will have reversing sensors.