You do need onward travel within 180 days on a DTV, it's classed as a tourist visa. You have to apply for the extention, it's not a given and there's been many reports of denials
You're incorrect. Thai Immigration requires onward travel within your period of allowed stay unless you have a long stay visa. The DTV is not considered long stay.
The TDAC is just an ARRIVAL card, there's no requirement to provide an outbound flight. However, if asked, it is a requirement to provide Thai Immigration with onward travel within the time frame of your visa