Online Non-O eVisa (it is not free, can not remember how much it was, not a significant money) is available to use 90 days after approval and the clock starts ticking when you land into Thai. On it, you can stay 90 days from the day of your arrival to Thai.
Changes to your fligt would be of no significance, don't even mentioned that when applying for Non-O eVisa.
You will need the exact flight date when you submit the TDAC (Thailand Digital Arrival Card) that you can submit online, free, gets approved within minutes. The TDAC you can do not more than 3 days prior to your arrival to Thai anyway.
So, with Non-O eVisa you are good, whenever it is approved, hard to imagine many more days. Then you have 90 days on that visa in Thai. Mine was approved in 4 business days (Osaka Thai Consulate).
If there was a problem with the documents, you would have been informed already.
With DTAC, you can have it printed on paper on your mobile phone.
However, Imm officer would even look at it, as soon as they swipe your passort, your DTAC pops up on their screen.