So, basically you were saying that they are similar rhythmically because of the simple time signatures
It's less about the simplicity of 4:4, it's more about how most conventional music works in 4:4. If you want an extreme example of this, listen to pretty much any dubstep song from back when it was popular. This isn't a dig at dubstep, it's just that it's an established 'rule' of sorts in there, for kick to be on the 1 and snare on the 3. This is because these are the strong beats, these are the ones that you bob your head down on.
The 1 and 3 being the strong beats still hold up in most pop/rock settings, so that's why you'll hear a lot of melodies start and/or end on the 1 and/or the 3. It's not even a conscious decision to do that most of the time, it's just what feels best.
So, basically you were saying that they are similar rhythmically because of the simple time signatures, but aren't the same because the progressions are different, but sound similar; because, the same notes of different scales act the same which will make it sound similar?
Yes
The melody's second chord (the 4th chord) is the 2nd note because these songs are diametric so they stay on one scale. This being the 9th would be useful when playing something not diametric, where you can transition to another scale. Here it's simplified and loops around. Right?
It's not that a 9th is more suitable for a non-diatonic key, it's that a 1, 3 & 5 are obvious and pleasant choices for pretty much any key that they're in. If the chord underneath the 9th were to go out of key, then it's suitability would completely depend on what the chord underneath it was, and how it resolved afterwards (what the chord and melody note after it were).
To show you a melody which breaks a lot of these conventions, yet is still listenable to someone without the background, take a listen to this song
https://www.youtube.com/watch?v=Ozr4KsZBTvQ
It breaks all the right conventions in all the right places, to be both interesting yet still sounding good and catchy. This is probably one of the only songs that I really love that I've introduced my girlfriend to and she hasn't immediately disliked it. Great band.