1.) on the Thai E-Visa Website that all Thai Visa applicants must use around the world, a Pop-Up Window will show for anyone that selects Vietnam as their current location and will list the various provinces that the Thai Embassy in Hanoi and Thai Consulate in Ho Chi Minh City have jurisdiction over....
2.) For the Thai Embassy and Consulate in Vietnam, only Visa/MasterCard are accepted on the Thai E–Visa website payment
Did those friends stay in Germany after submitting and making payment for their DTV Thai E–Visa application at the Thai Consulate in Frankfurt, or, at least in Europe throughout the processing time?
Congratulations on your DTV and Thank You for your report, but the Thai Embassy in Seoul's website is not particularly precise about what documents are needed for those who are not residents in South Korea and the Thai Embassy in Seoul's verbage seemed to imply that only residents of South Korea can apply there....
But, if one looks at the details for a Thai Tourist E–Visa (no details for DTV E–Visa), then it seems, from your experience, an Entry Stamp/sticker upon arrival into South Korea is acceptable for DTV applications of those not residents in South Korea....
Perhaps try contacting the Thai Consulate in Guangzhou and see if they can cancel your Tourist E–Visa that they issued (unlikely), but you never know....
if your Thai wife is technologically inclined, then perhaps have her pull up the คร.3 (Rose Bordered Marriage Certificate) colour scanned copy in the ThaiD App and download the colour copy from the ThaiD App (you would need a Thai ID Card in order to use/register with the ThaiD App by yourself).
The ThaiD App is from DOPA, Ministry of Interior and has BORA documents and information of Thai citizens, such as Tabien Baan (Thai Household Book), สูติบัตร (Thai Birth Certificate), Thai Marriage Certificate, etc.