No one knows why Khun Ja switched Nadech and Mario's roles. [Some] fans were making the 'assumption' that initially, Yaya wasn't suppose to be in here but in the end, Khun Ja casted her and figured Preawdao wouldn't be suitable with her so in order to pair her up with Nadech, they'll need to make som adjustments. Clearly that isn't the case because if Yadech weren't even contacted for the lakorn, they wouldn't dare give hints when they were on Ratree Samorsorn for GRGR. When some mad fan or should I say the same fan (lol) went and complained to Khun Ja about the switch, she tweeted back saying something along the lines of "I want my actors to be challenged". I don't exactly remember how she responded but she's implying that she knows whats best for her actors.
When you ask which character is better, there's really no answer. It depends on your preference and the way you look at things. It depends if you're willing to open up and study the character thoroughly. Basic differences between the two are that Takeshi is clam, reserved and intelligent. His biggest weakness was his love for Preawdao. Ryu is a playboy, he's more impulsive, talkative and playful. The most obvious common trait between the two is that they're both 'Ruk Diew Jai Diew' type guys, meaning one love one heart for only one girl. Ryu may have been a playboy but if anyone read Kittiya's translations then they know that it was all a plan. He didn't want people to know that Mayumi is his love interest or she'd be in a dangerous position so he pretended to not care for her but really, he secretly watched her all along. He only had Mayumi in his heart although he was a bit of an arse on how he approached her after the their years of seperation. Bottom line is every character has it's own uniqueness. It's up to the viewer to determine what they think is "better".
LOL! I don't think you need to worry at all about Yaya pulling off love scenes. I don't think that anyone should worry. 3 lakorns from Yadech have been more than enough to prove that these two are very capable and always have explosive chemistry. When one reacts, the other knows how to respond. Don't forget that Pajaew even said, during the underwater kiss in DJA Yaya was the one who agreed to do a real one because she understood that it was necessary. If anything, you should worry if Nadech will kiss properly for once. His forehead kisses are fine but it's about time he gives Yaya a proper kiss on the check because I've seen more than enough sniff kisses. Maybe I'm just being picky and demanding hahaha.. That's why I say I want them challenged. New characters, new acting styles, new vibe etc etc.