A tourist visa wouldn't benefit you. You would get 60 days when you entered and then that would be invalidated when you left 4 days later. Paying for a visa for 4 days would be pretty wasteful.
I'm a little confused. Which extension are you applying for? They will 100% not add an extra day for you, so you can forget about that option. But it's hard to say what options you might have because your post is confusing and doesn't include enough information like when you entered, when your entry stamp went until, what extension you are applying for and when it will go until.
That says ITINERARY, it does not anywhere say you need a ticket. The e-visa application for non-O just asks for flight information in and flight information out. A quick google flights search can get you that information. It's just ticking off a box because a non-O is only valid for 90 days, so you give a flight out within 90 days and done.
What is the game plan then? Is this just a temporary trip to Thailand? Otherwise the only option moving forward is going to be extensions, or returning back to the US every 2 years to apply for a new non-OA visa.
A visa grants you the full stay from the day you arrive. The day you arrive is counted as day 1 and you'll receive 89 additional days after that.
Departure information isn't mandatory for ANYONE on the TDAC. It's not a mandatory field. Don't touch it and it will remain not required. Touch it and it all becomes mandatory.
The advice is don't apply for a non-OA visa. Why are you applying for that? It's only useful in very specific circumstances and is not the proper visa for most people. The non-O is the proper visa for most people and doesn't have ANY of the requirements, just proof that you are over 50 and proof of funds, that's it.