I was trying to fill out the immigration form online here at inm.gob.mx and it wouldn't allow me to complete the form saying my expiration date is within 180 days of arrival. My passport is expiring in 3 months, so this is probably why.
Now my question is would I be able to still cross the border if I just showed up at the border and fill out the form on the spot?


Go get a renewed passport. Pay the fee for expedited service if you are in a hurry.