On completing the Form 1310, you would mark line C and complete Part II even if you are not court-appointed. That should not require a "fight with IRS". You are totally within the law to use the 1310 and file his final return and accept that refund of his.
As far as whether or not he was entitled to the stimulus checks, refer to this link:
Anyone who was alive for any part of 2020 is entitled for the first and second stimulus checks. Answer the stimulus questions in TurboTax and TurboTax will calculate the correct stimulus based on your answers.
The IRS says:
"An individual who died in 2020 or in 2021 and did not receive the full amount of the Economic Impact Payment may be eligible for the Recovery Rebate Credit if the individual met the eligibility requirements.
An individual who died prior to January 1, 2020, does not qualify for the Recovery Rebate Credit."
Refer to the following link:
Decedents and stimulus checks?
Related Resource
IRS info on correcting stimulus return problems
