first, does this sound like the most sensible way to reduce what I owe?
It could be, but it depends on your spouse's other income.
I am also the primary financial support for my stepson and have been for two years.
Once married, you have the same legal right to claim your stepson as a dependent as a biological parent. If you file a joint return, your son is your dependent, because your son lived with at least one spouse. If you filed separately, the son is not your dependent unless the son actually lived in your home.
Second, if I do file MFJ, how do I procure an ITIN for my husband since he does not have an SSN?
Include a W-7 application with your tax return and file by mail.
Third, would he still have to file US taxes, given that his income in 2022 will be under the minimum filing requirements?
If you file a tax return for any reason, you must report all your income, no matter how small. If you file a joint return with your spouse, you must report all your own income and all your spouse's income, no matter how small. The filing threshold is joint and based on your combined total income.
If you do not make the election to treat your spouse as a resident for tax purposes, then your spouse would file a non-resident tax return to only report US-sourced income. If they do not live or work in the US, they probably don't have US-sourced income, except for some rare exceptions.
