@cparke3 -
1) were you two both below $125k in 2020? (look at Line 11 of Form 1040) NYTimes reports that Dept of Education will use either 2020 or 2021 to determine eligibility so if 2020 was below 125k for each of you, you have some breathing room here to decide how to approach your 2021 tax return (and there is more time for more information to come out).
2) "sure about the tax rules" - that is why this has been such a "cluster" to determine how to exclude high income earners. Tax information is confidential at the IRS. They do not share with ANYONE or ANY organization - not even other parts of the federal government (see the fight to obtain Trump's tax return as just one example!). For the stimulus and child tax credits, the IRS itself were the ones doling out the money so they already had the data. But this situtaion is VERY different. Dept of Education can't simply ask the IRS for a data dump of 40 million returns to obtain tax information. DOE student servicers don't have the resources to ask 40mm people to access their tax transcripts and submit; that is why there will just be simple form and what I call "trust me". And certainly services can't process 40mm sets of tax transcripts before payments resume again on Jan 1. (and remember, PPP was a 'trust me' and that was a lot of money as well!).
3) amending spouses tax return to be part of a joint return isn't a problem. Folllow @Critter-3 advice above as @Critter-3 is a 30 year expert in this field. IRS won't process the amendment for 6-12 months, but again, should not be a problem for the student loan foregiveness as DOE will not have access to your tax return in any event, unless your provide it (i.e. they can't get it from the IRS).
4) New York Times:
