Licensing – Free to Use but Not Free to Modify or Redistribute

I would say the GPL (or AGPL) is probably the best license to cover most of those points. There's no existing license that covers all 4 entirely, and it's questionable whether you can actually enforce all those points, since they don't quite add up. For example, you want people to push modifications to you, but you still want the right to determine who can redistribute the work. This can only happen if you require that all committers hand over copyright of their modifications to you. This might dissuade some potential committers.

On you're third point, you are worried about other's redistributing, but you want people to submit their changes back. With the GPL, these are effectively the same thing - because distribution of modifications can be taken back into the main branch by you anyway. It is required that anyone distributing a modification does so in source form.

As for requiring that all extensions be redistributable, this conflicts with the idea of being used commercially (assuming you mean in proprietary software. It does not restrict commercial use though, provided the source is still distributed).

So long as you keep using the version of the software which was released under the MIT licence, you don't need to do anything: nobody, not even the copyright holder can revoke the licence you received.

Of course, if you want to update to the newer CC-licenced version of the software, you would have to comply with whatever terms that version of the software was released under - or in your case, pay the licencing fee.