He probably means he applied for it outside Thailand, then entered Thailand on visa exempt or something and checked his email while in Thailand and hey presto his DTV was in his inbox.
What might be an issue with the diver training route, the individual courses are I believe max of two weeks. Now if your not already a qualified diver and enrol for all the courses you can, then that might be an acceptable requirement. If you just applied for the first module, which is probably only five days, but don't know for sure, the Embassies might not accept it, plus if you did apply after booking the initial diving module only and they didn't accept it, then you lose whatever visa fee you've paid. Whichever Embassy or consulate you intend applying to, best to call or email and ask them first.Keep us posted please.
If plan1 only gives you 400,000 baht of cover for 38,000 baht, at your age I'd be inclined to have 400,000 lying in a bank account. Once you cross the 60 years old, then the premiums will start to rise.
If your nearest border is Nong Khai, then it'd cost you $40 USD= 1360 THB for a Laos visa at the border, then if you come straight back across the bridge there's no guarantee the IO will grant you another visa. If you paid for a Laos visa with Thai baht, good chance they'd charge you more than 1,360 THB. Better as some have advised to go to immigration office and get a thirty day extension, which would cost you 1,900 THB.