I don’t have a problem with this in principle—I think immigration restrictions in the US are unjustly restrictive. But I think there are many problems in practice. For example:
There are legal penalties for immigration marriage fraud, including 5 years in prison and a $250,000 fine.
Many EAs advise following the math when trying to improve welfare but caution against breaking any widely held social/moral/legal norms. Lying about the purpose of your marriage would certainly count as one of those norms.
While there might not be substantial monetary costs to marrying someone, there are social costs:
Without substantial time getting to know the immigrant you’re marrying, you might not be familiar with their personality—they might be abusive or mentally unwell.
You won’t be able to marry someone else who’s more fitting for you romantically.
People who know you might disapprove of your dishonesty.
You say that this “plainly pencils out as optimal,” but you don’t provide the penciling. I think a full accounting of this decision would show that’s it’s probably unwise.
I second Hay’s suggestion of making a more formal argument. The unstructured sections of this post made it unclear which propositions you took to support which.
I’d also note that your definition of “objectivity” at the beginning makes it trivially true that morality is sometimes subjective, since people are surely at least sometimes biased by their emotions when discussing morality.
An alternative definition of “objectivity” that is pretty standard within meta-ethics goes something like this: X is objective if it is not constitutively dependent on the attitudes/reactions of observers. The funniness of a comedian is subjective because it is constituted by how amused the comedian makes people feel. In contrast, the solidity of a table is objective because it does not depend on anyone’s reactions.