Which would be absolutely ludicrous, given that after returning to Thailand at CNX now you just swing by immigration next to the airport and file the TM30 then go home and sleep it off. All the way out to Promenada is a total PITA. ✈️✈️✈️💩

"Visa on arrival may not be extended."

Difference with Laos is there are no direct flights from India. Whereas there are direct (cheap) Air Asia flights from Delhi and Kolkata to KL.

Croatian needs to get tourist visa in advance, cannot get visa on arrival or visa exempt entry to Thailand. Suggestion, fly to Malaysia (Kuala Lumpur) where you can enter visa-free then apply for Single Entry Tourist Visa at the Thai Embassy.

by air doesn't count against the limit of two visa exempt LAND entries per year

"I then went to Vietnam and came back"

came back how/where?

Embassy closed Monday October 14, 2019

"It is for exactly three months"

That is the validity period, within which you must use it. You'll get a 60-day entry which can be extended 30 more at Thai immigration for 1900 baht. Then you could do a border hop at Mae Sai / Tachilek for a 30-day visa-exempt entry (allowed to do twice per calendar year if by land border). No need to show proof of onward travel in that case.

You mean like SETV METV NonB NonO/O-A/O-X NonEd, etc.

And if your application is denied FUBAR SOL SNAFU ;-)

If online it has to be from 15 days before until 7 days BEFORE not after.