They said it will happen by the end of 2015.
Still waiting.
If you followed Chris Lattner on the dev forums he always gave the impression that they wanted it open source from the start but had bigger fish to fry. They reimplemented a ton of the compiler multiple times after finding bugs in the existing language specs. Don't read too far into this that microsoft forced anything. For one, we'll never know and this is at best conjecture.
As for supporting diverse execution environments, I'd argue llvm/clang/webkit proves otherwise. Granted they're not "supporting" it in the sense that they're selling support for it but I'm not sure exactly who would meet your criteria right now.
I think most people involved in software would realize that the distinction between plan and implementation is extremely important.
When I first saw this thread, I certainly thought Apple had actually open-sourced Swift, as both the headline and the comment I was replying to said so. Then I looked at the linked page and saw it had not happened, so I corrected this materially important piece of information.
Honestly if you're that nervous and scared of what might happen in the future there's probably nothing Apple (and most other companies) can possibly do to comfort you. Probably better for you to avoid the potential pain and skip this one.