Other legal concerns I should think of?

Getting sued by your employer for misrepresentation, fraudulent inducement, and making them liable for foreign payroll taxes? That would be my biggest worry.

Your history is irrelevant. Your child will have double U.S. and Colombian citizenship as long as you do the paperwork.

You only need one passport to leave or enter a country.

Google Fi is $10/month if you don’t use data.

You can only apply for the expedited nationality after two years’ legal residency in Spain. So you would have to apply for and obtain a visa. It takes years after applying to actually obtain nationality, but after you wait one year, you can hire an attorney to file for an immediate response.