This is very boring.
The thread you refer to about having funds on your person for visa exempt entry, I was correcting you.
10k for individual and 20k family
Also that it had to be in cash.
As for onward flight I always state that onward flight may be asked for by airline.
So does our expert ubonjoe.
As for the OP he has 2 options.
Go to immigration and request an extension and when denied will be given 7 days to exit Thailand and pay 1900 for application.
Other choice is to exit Thailand with one day overstay.
In most likelihood he will not be fined. Has nothing to do with showing delayed flight.
I also note several others have suggested to just exit, uj being one.