There are good reasons to argue that the 2015 SPACE Act is consistent with the 1967 Outer Space Treaty.
Assigning ownership certainly looks like an act of sovereignty to me. After all presumably the USA would try and enforce the property rights that it had given. If there are laws that allow the USA to requisition private property this would seem to further exacerbate the issue.
While this might be an attempt to set CIL it fails in and of itself as it is one nations law. If they actually where interested in that they should have got an agreement with other nations. A treaty between other nations might come from this, and therefore from one point of view being successful, but it might be quite different if just to show not being under the USAs thumb.
Of course another option is that other nations just pass there own laws with the result of different people claim different rights on the same thing. After all it would be just as valid for another nation to make its own incompatible law. An extreme example is that it could just assign random mineral rights to individuals and as far as I can see that would be just as valid as the USA's law.
In other cases, other nations have adopted US law as a template for their laws. If enough countries do this, you have CIL. This is part of the intention here.
Think a little about how this might play out. I, a citizen of one country, send a spaceship to asteroid X and start doing stuff. As long as I am there, you (a citizen of another country) are not allowed to interfere with my operations. Now, interfere can mean a lot of things (is it interfering to land 100 m away? How about 100 km away?), and that will have to be worked out, but the principle seems pretty clear - and that is true whether I am doing geology or astronomy or mining unobtanium. You don't have the right to interfere with my spacecraft going on to Mars, or back to Earth, or wherever, either, whether it is carrying unobtanium or not. All of this seems quite settled under the 67 OST. Also quite clear is that, once I leave, you are free to step in and do your own thing too. (Now, I may leave behind monitors or something, and again, what it means to interfere with them will have to be worked out, but, again, all of this seems quite settled in principle under the OST.) So, I do not have "mineral rights," just a right to operate.
All of that seems to be clear whether I am doing commercial work, or science, or something else. And, if I bring stuff back, it is also quite clear (I would argue clearly CIL) that the country whose flag I am under gets to decide what's done with that stuff. They can say I own it, it belongs to "the Crown" or the people, I have to pay tax, or whatever - that's up to to the national government. That's also quite settled under the OST.
One thing national governments can't do is say "You now own the Moon" (or Ceres or wherever). I.e., they cannot "assign random mineral rights" (or, at least, expect to have other countries abide by that). Note that this is one thing that the 2015 SPACE Act does not do.