OP, sorry to hear of your dad's health issues
My first thought when I read your list was that the answer was basically no to each of your questions. But not being a tax or accounting expert myself I was hoping that more knowledgeable posters would chime in.
I've operated similarly to your father for many years, a small time contractor on a cash accounting basis, so I do have a little insight there.
I've often "cheated" a little, like if I do a job in 2024 and all the expenses are in 2024, but I'm paid in 2025, I will move the payment back to '24 for tax reasons. But that's kind of the opposite of what you're dealing with.
I'm assuming your father is still in contact with the client, and it sounds like he wants to do right by them and finish the job by hiring subs to do so? Is the relationship still solid? Are they understanding?
If so , is there enough money left to pay subs to finish the job?
Can your father just write a check and return the balance to the clients?
When you are a schedule C filer, no one is checking dates or really looking that closely at your return, unless you are audited.
I would be sorely tempted to have your father write a check dated 2024 sending the balance of the money back to the clients. Then the clients could write a check to your father now in 2025 if they/your father wanted to try to finish the job. Essentially this is your question 3 & 5. Also, to question 1, the issue is if you have to send 1099's to any of the subs that would be a red flag to the IRS.
I think it's justifiable, given the situation, but it's likely the IRS would see it differently.
Really, though, it would probably be best for your father to give up on the idea of finishing the job and just focus on his health and family.
Perhaps a silver lining is that the recent purge of IRS workers makes the chance of an audit less likely.