Once a dtv is issued you don't need to "activate it" for it to work, it already does. The only thing you are doing when leaving the country is obtaining the 180 day stamp. So yes leave Thailand whenever you want, before your current tourist stamp expires.

Same thing happened to me. Dtv approved 2 hrs after I arrived in Thailand. I didn't leave Thailand until 50days later and returned on the dtv. No issues at all