Also, just having canceled the visa would not have done the trick. You entered before the DTV was issued. So, if you had canceled and returned, you would most likely have entered with a visa exemption (60 days). You still would have needed to exit and re-enter to activate the DTV and get off of the visa exemption.
- Not every embassy website has this language, but many do. I think it makes it much clearer. The Ministry of Foreign Affairs (based outside of Thailand at embassies/consulates) is the only entity that supplies visas, and they are used to enter the country (hence why people are saying you need to exit and re-enter with the DTV). Immigration (which is based inside Thailand) can supply you an extension of stay (we often just call these visas too, but technically they are not). Immigration can not issue DTVs and can only extend them for another 180 days at the end of first 180 days.
Also, for clarity, you can only extend your stay one time per entry. So if you entered today, your stamp is for 180 days. You can then extend for another 180 days, but then after that you have to leave. You then can re-enter and repeat.
Not sure which embassy/consulate you applied through (4 in the US) but this is what has been posted on the DC's embassy webpage. 15 business days, but you should plan for a month. Where did you see 7 days?