I would recommend Windows 10 and a FreeSync monitor to make the most of DX12. Hopefully the community can help you with the other questions, as I'm not able to talk about unreleased products.
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AM4 mobos should start shipping within a month based on industry reports. All of the major mobo makers should have various model AM4 mobos shipping by Q4 of '16 when Summit Ridge CPUs are scheduled to start shipping. It will take some time to fill the channel so be patient. Based on the tidbits AMD has released Summit Ridge will definitely deliver the performance that AMD enthusiasts have been waiting for.
The issue with Win7-8 support is undecided at the moment but the pressure on MS is mounting daily. They have already had to rescind their update refusal on Win7-8 on Intel's Kaby Lake so Zen based products should be next. MS will be in court for years for illegally updating enterprise PCs to Win10 without any authorization. Technically Win10 has some advantages over Win7-8 but many consumers are refusing Win10 due to MS spyware, tracking and encrypted reporting amongst other issues like code defects, update issues, etc.