For me is interesting how and why BMW lost its lead to MB, so I tried to figure it out for myself. And the big reason for me is MINI. We've been told that it is a huge success, that it brings new customers to the group and so on. But such a project involves huge amount of resources - engineers, production facilities etc. I remember reading that BMW was hoping to break even somewhere around the end of the lifecycle of the second generation. So for those 14 years, if all those resources were poured into the core brand they would generated not only bigger profits but also better market position for BMW.
Then the third generation came. To make it profitable (the other opportunity is just to cease to exist) BMW had to achieve economies of scale and they started diluting the BMW brand. One may say that the new X1 sales better than the old RWD based one, but for me the reason is not the switch to FWD platform, but better design appealing to more people. I have never read a direct comparison of the new one and the old one. I bet that the old one is much better (of course its design is its biggest drawback). For sure, the technology of the drivetrain of the old one is superior to the new one and if only they had made this design mated to the old technology, the car would be even a bigger hit. So, BMW is forces to use its FWD technology, because it already invested huge amounts of money in it. I don't want to mention anything about 2AT, but having the future 1 series in FWD is just for the reason to keep MINI alive.
And now, the biggest problem is coming, one that no one could predict, the Brexit. It will make all MINI products noncompetitive, the margins will go into the pit and BMW will be left with heavy problems to solve.
This is just my opinion
I disagree with most of your conclusions, but you're right that Mini was not projected to have returned it's initial investment till gen 3, but the fact is, that investment is now paying off.
Long term growth requires investment, BMW could have invested those resources into its core models, but where would the growth be? They're not going to start selling double the number of 3 series because they invested more in it, the target market is only so big. And yes they could have invested in niche top end models earlier, but they won't show the kind of unit growth required to make headlines.
In developing the Mini brand BMW now have an additional 300-350,000 sales per year, they have economy of scale on a platform which allows BMW to increase its own brand sales, whilst being profitable, in a way they couldn't do otherwise. Thats 350,000 sales that didn't go to Daimler, VWAG, FCA, Ford, Toyota, Peugeot-Citroen, Renault-Nissan or Honda.
You might not like FWD, but BMW could not show significant growth without using it. They have to have a sub 3 series platform on which to provide the large volume "cheap" end of the market, and the more models they can spread it across the better. This is where the higher volume exists.
As for Brexit, our currency has dropped, so BMW may well make
more money, not less - the risk is in tariffs and taxes, but we're potentially a couple of years off this, and it might not happen. Mini is not the only risk in that scenario either, Rolls-Royce would suffer as would the Hams Hall engine plant which supplies the BMW Group. Add to that, Mini is not solely reliant on the UK for production as variants are also made my Magna in Austria and VDL Nedcar in the Netherlands.
I respect your opinion to not like
how BMW is doing things, I enjoy my RWD 1-er

-- but the fact is your first question was how and why they lost the sales lead to Mercedes... and I don't see how not having Mini and removing FWD architecture would allow BMW to grow faster.