Let's look at this statement.
Marketing can't make a case if the cars don't sell to begin with.
Who decided on the price and it's placement in relation to the competition and further more who decided on the potential quantities needed in any given year.......
the engineers? 
It's the marketing department that looks at the competition, sees what is needed, evaluates how much can be charged which into decided the profit being made and guesstimates the potential numbers that might be sold.
As a matter of fact, yes, the engineers can have a huge say in how much it will ultimately cost. When you say marketing and business case are intertwined, you are correct. But I don't think you are correct in saying that marketing
alone determines whether the business case can be made. It involves numbers crunching by marketing, engineers (who, after all, are ultimately the ones responsible for determining the level of R&D needed to even make the RHD case from which calculations of cost/benefit can be derived), manufacturing (what is needed on the plant floor to allow the change, and will it adversely affect capacity elsewhere?) and no doubt input from finance (bean counters) who oversee the budgets of
each of these departments themselves.
We would have know a little more about the F10 platform and what would be needed to make the business case for an AWD version with RHD. Perhaps, as like Mercedes with the GLK, they felt that AWD and RHD was not feasible at the time of its release and they didn't want to foot the bill for R&D until later.
As for how Audi can do so well with AWD in the UK, well, their identity has been forged for decades with a close association to AWD. Those rally championships weren't just a result of marketing; it was the result of good engineering. Without the latter, you can't have the former (as Cadillac found to their dismay after their failed Le Mans program). Meanwhile, BMW has traditionally been based around RWD. This is certainly changing now, but at this point, it's a weaker part of their portfolio than it is for Audi. And according to Audi's Wolfgang Egger, whether a product fits a company's portfolio can be the #1 priority over considerations of cost, engineering, volume, etc.
The problem with relying solely on marketing is that it gives only a snapshot in time. No doubt Porsche and Mercedes did considerable marketing drives to potential customers with the CGT and SLR, but as they found out, interest was strong initially, then waned and in the final years of production, sales slowed to a crawl. Both fell short of their intended projections. Same thing with the Z8: it sold very strongly at first, but sales trickled in the final years. Throughout this, the cost of engineering pretty much remains the same and thanks to the changing nature of the public, a company can find itself with a hefty cost in engineering but no more customers. It would be nice if marketing can accurately gauge interest, ensure to collect deposits on all cars beforehand and miraculously work with manufacturing to profitably build all cars within a short period of time, but sadly it doesn't work that way for this end of the market.
My complaint is that because of this the M550 won't be the car it should be when it arrives here if it arrives at all.
