Microsoft needs to do two things.
1) Give the mobile group vastly more independence to escape the dying desktop business.
2) Make Windows Phone compatible with Android applications. Bribe every major hardware vendor to put out a big new line of Windows Phones to launch with said compatibility. There would no doubt be big tradeoffs in doing this, but it'd be worth it, and would make use of the classic Microsoft strategy of embrace and extend that they seem to have lost an appreciation for after dominating for so long. It's a trojan horse approach superior to their goofy dual booting idea.
I see no reason Microsoft couldn't acquire 20% of the mobile market. Their currently mediocre strategy has delivered some success in Europe, so imagine what a good strategy could deliver. The real question is, can owning 20% of mobile ever be worth the cost if you don't own the hardware (Apple & Samsung)? I don't think so.