Selection of candidates:
I strongly suspect that none of the three judges had any say in the selection (with the exception of Smarmy and Clueless who must have been pre-selected

). All of the other couples were chosen by the production team purely for how they would come across on TV, and I would guess that the selection process did not even include a requirement that anyone should be able to demonstrate basic cooking skills
The format:The main change I would make would be simply to take the format back to how it was in the first two series, with two separate hour-long shows consisting of the initial challenge and analysis and then the bottom two challenge. I would definitely bring back the analysis at the end of the first hour when each of the couples was seen individually and told what their strengths and weaknesses were and what aspect they must concentrate on in the next task.
I would also make sure the tasks were relevant. This year there seemed to be no mention at all of budget, profit or turnover. Giving a cookery demonstration at a food fair was a ridiculous task to impose, because while doing something like that might be a lucrative sideline for a restaurateur it is certainly not a prerequisite of running a restaurant. This year it appeared that the only requirement for success was that they should be able to blag themselves out of doing whatever they were required to do.
One change I would bring in (stolen from the US show The Chopping Block) would be that instead of each couple having their own restaurant there would be only two restaurants. Four couples must work together as a team to run their restaurant. Each week the restaurants were judged by genuine food critics (another good idea) and then one couple from the losing restaurant would be sent home. I think doing it this way one can really judge their leadership skills and also team-working skills.