Lani6, regarding your thought #2, perhaps you misunderstood this part my post that you referenced. It does say that having taxes withheld instead of making estimated tax payments can help you avoid needing to file Form 2210.
As for the default treatment of tax withholding as being received evenly throughout the year, this is in the instructions for Form 2210 as VolvoGirl said. It's not just an IRS rule but is mandated by statute, section 6654(g) of the tax code: https://www.law.cornell.edu/uscode/text/26/6654#g
