I was thinking about your description of falang. In many ways it is the same as the Mexican term 'Gringo'. 200 years ago, Gringo was used for blonde, blue eyed men who lived near the border. Over the years, it gradually began to include all light skinned men from the north, usually Canada and the USA.
From the 15th century to today, the US has allowed large scale immigration. Blacks, Whites and Asians have fought in every American war. Intermarriage is a fact of life. Because the US birth rate has been decreasing over the last 50 years, Americans have been adopting foreign children. It is fairly common to see children that are a different ethnicity than their parents. My son looks Asian, but I look Caucasian. He thinks of himself as an American, a Gringo, a falang. I have an associate who was born Vietnamese, but adopted by white Americans. He also thinks and acts like a falang.
There are many 'falang' children in Udon, who speak Thai and think Thai. Most are half Thai, but some have foreign parents. Are they falang? A few weeks ago, 2 Thai men came into a shop with their blonde wives. Both men spoke English in the California style and seemed to have difficulty speaking Thai. Born here and raised in the USA. Are they falang?
Also, we cannot forget the people on Thai TV. Those men and women are whiter than I am. Some are obviously half Thai (luuk krung), some have parents that are also half Thai, but many people don't realize this. 'Bird" Mac-In-tai (MacIntyre) is a popular Thai singer. I mentioned to my step daughter that MacIntyre is a Scottish name. She was startled and insisted that Mac-In-Tai is a Thai name. Nobody calls Thai celebrities 'falang', no matter how much they look falang.
So, where do we draw the line? By birth, by appearance, by language, or by a combination of those?
Anyone who is half falang and looks more like a falang is considered a falang regardless if they speak Thai or think Thai.
McIntyre is definately a falang name, and not a Thai name. It depends how you pronounce it, Lao and Thai people say "McIntai" so his name sounds more like Thai, but if you say McIntyre then it's a falang name. Thongchai McIntyre is half Thai-American and alot of Thais don't know that because he looks more Thai than caucasian, so people think of him as Thai. He is actually gay but Thai people try to avoid talking about his sexuality because they don't want to give their superstar a bad image or they don't want to be impolite.
A persons appearance is the main feature that determines what Thai people call them. Tata Young is another popular Thai celebrity who is half Vietnamese-American, but raised in Thailand so everyone thinks she's Thai-American. She looks more like a falang than an asian.
I think only their fans don't call Thai celebrities falang even if they look falang, probably because they consider the celebrities as one of their own Thai people. Thailand is a multicultural society, and there are many mixed people such as Thai-Chinese, Thai-Falang, Thai-Isaan so they just call them Thai.