Is it acceptable to make my non dep 22yo with a substantially lower income 100% responsible for the shared marketplace policy that he is listed on with my husband and I?

Yes, it is acceptable to allocate 100% of the policy to your lower income dependent. In fact, this is the most tax efficient allocation if you all agree.

I was just concerned as it makes a significant difference in tax responsibility for both of us to our advantage of course.  Do you know if this flags for review since I was the one who applied, not my son.  Is this just something that is available due to loose wording in the tax law?

The tax law allows to allocate whatever percentages you agree upon to each taxpayer when you share a Marketplace policy. Allocating the policy to the lowest earner will give you the highest premium tax credit.