Is it acceptable to "white out" the incorrect AGI and hand-write the correct number in its place - and then mail in a copy of that form they said was "official"?

It is never acceptable to "white-out" any information on any kind of official or legal documents.  If you want to alter a document, mark a line through the incorrect information, write in the correct information above or next to it the correct information and initial the changes.  The incorrect figure should be perceptible.

However, the previous years' AGI is for identification purposes of the IRS to allow you to e-file.  Since you are not e-filing, it is not important.

The AGI that was incorrect was not the AGI for 2017.  The 2017 AGI does not need to be corrected.

Can you white out on the same form the irs sent back because you signed your name in the wrong spot and send the same form back