Cite? My husband’s prior employer didn’t offer either, as far as he knew (then again, he wasn’t in their health insurance because mine was better and cheaper). I admit I could be wrong about it being mandatory, of course.
That’s a separate thing, however, from the health insurance premium the OP was asking about.
The flex spending is also better than planning on deducting your out-of-pocket, for the same “must itemize, only amounts over 7.5%” rules. The danger being, of course, that you overwithhold and wind up losing the money at the end of the year.
I understand that your box 1 and 3 would differ by the D. pretax 401(k), but why would it differ by taxable insurance premiums? I’d think those amounts would be included in box 1 taxable income and perhaps noted in Box 14.