No it isn't.. the T8 form is now MANDATORY for departures from the 45 yellow fever countries. You're not getting on a flight from those countries having not filled out the form. Period! Airlines are checking this.
You might need to show a return or onward ticket to the airline before getting your boarding pass issued. I HAVE seen Vietnamese immigration ask for proof of outbound travel for those entering on the 45 day visa exemption as well, but you're most likely to be asked at check-in, in Bangkok (or wherever you fly from).
If you're concerned - then travel by land. No need to show an outbound ticket that way.
From Bangkok, the fastest way is via a cross border bus to Phnom Penh, then staying a night before catching another cross-border bus to Saigon.
From PP to Saigon, there are at least 30-40 daily round-trip services each day with almost a dozen different operators. From Bangkok to PP, there are just 2 or 3 daily round-trip services via 2 or 3 different operators.
You'll qualify if you have 500,000 Baht in your bank account. I don't think $400 is enough to live in Thailand every month but if your parents send you some extra money for support and/or you also have some passive income, such as rental income from back home coming in, it should work. That would mean probably a bare minimum of US$1000 / month plus some savings in case of an accident / illness plus you'll want to take out a travel insurance policy.
Alternatively, since you're only 19, you would be better off studying, learning a trade, working and saving up to perhaps eventually come over to Thailand but not now, maybe in 5+ years once you have saved enough, have a qualification etc.
In the meantime, nothing stopping you from coming over and visiting Thailand for a month or two, but at 19, you're not ready to settle down here yet.
And no, checks are NOT random. It's an absolute mandatory requirement for arrivals from the 45 yellow fever countries to be vaccinated for yellow fever to enter Thailand. Nowadays, they're doing the checks through the website/app, rather than checking again on arrival. The on arrival check may still be conducted if you're a national of a yellow fever country not arriving from a yellow fever area.
In such cases, technically, the requirement shouldn't apply, but citizens of these 45 countries may still be hassled regardless.
No, the T8 (OK just checked not TM8) is mandatory for all flights departing from yellow fever regions heading for Thailand. The rules came into effect last August. You have to fill it out or you won't be issued a boarding pass.
Had your wife traveled directly from Latin America, she would have needed to fill out the TM8 form.
However, how did they pick her out from the queue, having arrived from New Delhi? Normally, flights that don't originate in Latin America or Africa (there aren't any direct flights from the former anyway) shouldn't have anyone waiting for you to direct you to the quarantine counter unless you're already in line, or based on your appearance, immigration officers call out "black guy" or "Latino" (in Thai) and tell you (in English): "you go see doctor".
I've seen that happen. Several African men were pulled out of line and asked to go to the quarantine counter.