"Accrual" on first 2019 Sch C by accident...Can we amend to be "Cash" and not need to file Form 3115?

[SaraW]

I have a new client this year that (unfortunately) filed a DIY return for 2019.  She started a bookkeeping practice in 2019 and checked "accrual" instead of cash on her first Sch C (probably because as a bookkeeper, you're mostly thinking in accrual terms).  She wasn't aware that was a "big deal" for tax purposes until after filing, she received a notice from the IRS that more info was needed to process her 2019 return and got some outside help (me) to look over her originally filed return.  We gathered up the requested info, she sent it into the IRS with explanations of changes that should be made, and after 7 months of supplying that info to them the return is still "in-process."  With the info she sent in, she did send in a Sch C of what "should have been filed" and that was marked as "cash."  

So now I feel like we're kind of in limbo for 2020...Without knowing what the IRS is going to process/accept for that first Sch C, do I run with "cash" because that's what should have been checked or do I file a Form 3115...which seems overkill in her case since the net change would be (at least from 2019 into 2020) minimal.  Only one payment from a client of hers is causing a difference between accrual and cash basis. 

Also, if I run with "cash" now and if after filing a 2020 return, her 2019 return processes with "accrual," would I simply be able to go back and amend her 2019 return to reflect "cash" and then all would be square?  Honestly, I'm at a loss right now and am open to any ideas/suggestions from anyone who's experienced a situation like this.  Thank you in advance!!!