1. I'm inclined to just 'estimate' the amounts for 2017-19 using numbers a few percentage higher than the previous year (starting with the known numbers from 2016). Is this an acceptable approach? The investments are primarily mutual funds. Not a wise approach ... it is best to be more accurate. If you know the brokers then see if you can set up online access to the accounts to get the missing information. If you have not yet done so you need to get a Power of Attorney so you can speak to them ... seek an elder attorney to set this up and get her affairs in order.
2. Is there a higher risk of audit if I use the approach in #1? At this point I would think the IRS would be thrilled to get any return/payment at all. Since you have until 4/15 to mail in the 2017 return it would be best to get the information first. Incorrect or missing info can trigger an audit.
3. What should I expect from the IRS transcripts? Will they have the information I need to complete the tax returns? Note that they'll be sending these to my mother's address, so there's a chance they will get misplaced (she is older and sometimes just throws things away). Income Transcripts take a couple of weeks to come in and they will have the federal info on them but not the state. Put in a change of address form with the IRS and state ASAP and also have her mail forwarded to your address since she cannot be trusted. https://www.irs.gov/pub/irs-pdf/f8822.pdf
