Ouch....This is one that could take up most of your summer and fall.
Are you sure the corp was operating from 2012 until date of death? If so, I think you have a bigger issue than the depreciation schedules. Do you have income statements for all those years? If not, the books need to be produced (can you get bank statements for the years in question?).
It may be that you can work out a deal with the IRS, but if it is a CA Corp, I don't think you are going to get out of all of the minimum corp franchise fees for all those years, and CA penalties. If the corp was operating and did not file, there is a big mess to fix. And the Corp has to be collapsed with the CA SOS to stop the fees from continuing to accumulate.
Good luck my friend. I hope the kids have the resources to pay for the clean up of the mess their folks left them.
Tom
Longview, TX