Did you evaluate the possibility of requiring a signed CLA (Contributors License Agreement) for contributions? I think that way you could set the terms under which contributions can be made and include under which license everything falls and that you have the full rights to the contribution etc.
Many companies and foundations that maintain open source software require contributors to sign such CLAs, like the Eclipse Foundation, Canonical, Google, Microsoft and others.
I think most open source CLAs have the purpose that the maintainers can be sure that contributors grant rights to the end-user and also allow project sublicensing, but I assume that can be modified to your needs.
This might require some more research if you want to be on the safe side.
Did you evaluate the possibility of requiring a signed CLA (Contributors License Agreement) for contributions? I think that way you could set the terms under which contributions can be made and include under which license everything falls and that you have the full rights to the contribution etc.
Many companies and foundations that maintain open source software require contributors to sign such CLAs, like the Eclipse Foundation, Canonical, Google, Microsoft and others.
I think most open source CLAs have the purpose that the maintainers can be sure that contributors grant rights to the end-user and also allow project sublicensing, but I assume that can be modified to your needs.
This might require some more research if you want to be on the safe side.