Or can I use the alternate valuation date that my sister as executor used? Yes, it is close enough in time that not much would have changed.
Also, do I need to issue 1099-NEC's for painting to get house ready for rent, etc.? As you are not a business but an individual, no. If your rental rises to the level of a business, then yes.
House would have been ready to rent in March/could not work on the house again until May. Which was it? If the house was ready and available for rent, why did you continue to work on it in May. Deductions start when the house is placed in service and available for rent.
Is there a tax break for that lost income? There is no such thing as lost income. You can't lose what was never yours to begin with.
or do I have to list every day in 2020 until the renters moved in as personal-use days? No, these are not personal use days unless your were living there. It is simply in a state of limbo until it is placed in service. At that point, even if you can't find a renter, expenses are deductible.
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