I think you will be fine. If they pull you aside to ask more questions, play it cool. Explain how much you love Thailand (as you did in this post), explain your employment/life doesn't allow you to live their full time, but you enjoy spending your free time in Thailand. The immigration officers like hearing good things about their country.
I think people run into problems for one of two reasons, A) They're assholes who think they are entitled so they become combative, or B) they have a problematic history (black listed, then arrested, then ....).
You seem like a decent bloke who spends money in Thailand, you're exactly the kind of people they want in their country.
If you wait until the last day, you run the risk of not having all the necessary paper-work, then getting denied.
Usually you can a extension 14 days ahead of time. Each immigration office is a bit different on the 14 days, so really it's a matter of which immigration office you use. I believe BKK is 14 days.
I don't think it matters too much. In both cases, when you stamp in, you get 60+30, then you will have to exit and return. The METV saves you the hassle of re-applying every 90 days, but it's not necessary to get a visa (depending on your nationality). Hence, you're just throwing-away money but getting the METV.
I am doubtful they will reach some quota within this time-frame. I have never heard of Thailand implementing quotas, they may, but I don't think its very common. Thailand benefits from having visitors.
Vietnam has goofy visa rules, probably more goofy for Americans (they call it the American war).
I haven't been to Vietnam in 5+ years so my experience is out of date.. However, from what I recall, you will need to travel with crisp brand new US $20 to pay for anything government related.
Basically you buy a invite letter.. There are services to that provide this. You get the invite letter, print it out, take it with you. Once you arrive (before passport control) you go to an immigration office with the letter and crisp brand new $20's and buy your visa.
If you need to get a new Thai visa, you will also have to pay for that with crisp brand new $20's..
As I mentioned, things may have changed. If I were you I would search for "I am [your nationality] what are the requirements to visit Thailand" and do the same for your girlfriend, most likely her requirements are different.
"Only 1 question asked: "Why don't you apply in your home country?""
That's very interesting.. Many people in this group think you can only apply in the country you are currently in. But Jakarta thinks you should apply in your home country.
Do you mind me asking what is your nationality (where is your passport from) ?