Are you over-complicating things? I think you might be.
I'm going by US law here, I think though that many international laws are similar:
HERE IS A FIRST OPTION: EXPLICIT APPROVAL
Holding the following true:
(1) Someone, by default, owns the copyright to the source code in question. Copyright ownership is automatically attributed at the time of creation, to the creator.
(2) The copyright owner (let's say John Smith) is the sole controller of the right to reproduce what is owned. All rights are granted -- or not -- by the guy who owns the copyright.
(3) If John Smith says it's okay to use his source code for for-profit purposes under circumstances a, b, and c, then it's okay to use the source code for for-profit purposes under circumstances a, b, and c. John Smith said so. He's the owner.
(4) Get John Smith to say it's okay.
Step (4). That's it.
HERE IS A SECOND OPTION: DECLARED PUBLIC DOMAIN
Another avenue is to make it public property. Our kind-of-spirit Mr. Smith can say, "I now release all copyright ownership. Period. This is now Public Domain material." Then nobody owns it (or to be more accurate,
everybody owns it!) and everyone can use it, for whatever they want to, forever. A potential drawback is that this is a one-way street (unlike trademarks, which can lapse into public domain and then be made proprietary again). Once ownership is put in the public domain it stays there. But you'll never have to worry about any of this legal nonsense, because if someone disputes whether it's public domain? At worst, ownership reverts back to John Smith. Who can then waive every case of use, making said dispute meaningless.
It's public property. Anyone can use it.
Two options that could make this simple. I say, make it Public Domain and be done with this. It's a waste of time, when you should be coding MH so I can get my darn mesh with an articulated jaw that opens and closes