MAME is the pen name of LBC and TharnType's writer, she uses it in capital letters that's why we also write it like that.
Sam is Thorn best friend and Tharn's first time, the guy who came and said things to Type to make him jealous, the one who kissed Tharn just when Type was watching, remember?!?! He appeared on episodes 8 and 9.
Tum and Tar are stepbrothers, but Tum has always seen Tar as more than a brother, even when Tar was dating Tharn(OMG, too many names starting with T in this sentence lol).
So, you want to audition as Khom's couple?!? I think first you should ask MAME or tell her exactly what you told me XD, or you've to wait for the series to be announced(well, if it is a series and not a movie), then travel to Thailand and go to the auditions!!! As simple as that!!!!
Thorn and Lhong.... I don't know, I see both of them as bottoms, Thorn for me is kind of like Type(3 years later Type) but a lot more mature and calm, you know, a bottom that stands his ground and has his own voice, if you get what I mean. On the other hand Lhong, I don't know, I can't imagine him being with someone, maybe because in the series he was so obsessed with Tharn that is difficult for me to picture him in a relationship.