not necessarily. Some embassies have it clearly stated on their website that you must remain in country until your visa is received. Embassies are starting to ask people to come in for interviews, want copies of all passport pages again and other weird little requirements that are changing daily. The only thing using e-visa system does is change the way you submit info and you don't have to hand in your passport. It doesn't change the fact that people are "expected" to remain in the country they applied to for the duration of the process.
Have people left countries before getting receiving the approval? Yes, some have been successful doing this but more importantly others have been denied and lost the visa fee. It's not as cut and dried as you suggest. The OP should know there are risks associated with leaving.
well there is what this individual is going through that was posted 17 hours ago in this group. There are posts in the DTV group about people being asked in for interviews and for copies of passport pages. One individual from Poland previously posted issues in September... the information is out there if you search for it. Some embassies like South Korea state you must remain until the visa is issued on their website as have others. There is also the note on the evisa site under the country you select, photo attached.
Anonymous participant was the Smart visa issued by a special area like BOI, or just regular immigration? If through BOI I'd ask them. You could also try asking the embassy you'll apply at. I don't know enough about the Smart visa myself, is it multi entry? It does seem that people with an existing METV have to wait till it's expired. People on work visas and I think an Ed visa have been able to cancel it after returning so not sure if it works like that. Maybe ask in the Thai Visa Advice Q&A. There may be someone there with experience.
You need to leave Thailand to apply. When you log into the evisa system it has a pop up stating you can't apply for a visa while in Thailand. Pick a nearby embassy to apply, search the website of where you choose and posts to see requirements.
Print is easiest, saves having to hand your phone over to the immigration officer. The e-visa system has a pop up that tells you to print the visa to hand to immigration but people have been successfull showing it on the phone. Either way, you need to make the IO aware you have a DTV and check your dates before you leave to make sure you got 180 days.