Do estate tax returns default to calendar year end tax years if fiscal year was never elected?

I have a lawyer who keeps sending me estates, asking to prepare the first and final return, but the decedent has normally died 5-8 years ago. No income or expenses had happened in those years prior.

Real world example:

Decedent died May 22, 2018. No income and expenses until April 24, 2024 when the house was sold. Attorney fees and other expenses were then incurred. Obviously my CPA fee has not occurred beyond a year after income first occurred.

1. Am I forced to file calendar year, filing a 2024 return late, as this would have been due April 15, 2025? Unfortunately all expenses wouldn't be deductible since most were in 2025 (attorney and cpa fees)

2. Can I assume a fiscal year end was elected and file May 1 – April 30? If the IRS questioned which I doubt they would, I would back file however many $0 dollar returns were needed to show the original election made.

2.5. Wild what if question based on the above – can I assume a partial fiscal year end in the first year? First year return was May 22 2018 – August 31, 2018. Second year return September 1 2018 to August 31 2019, etc. There has been situations where that scenario would allow the sale of the home to line up with all fees.