Sí, la oración "Javier me dijo que llegaría la semana que viene" es correcta, también podrías usar otras formas verbales como "Javier me dijo que llegará la semana que viene", pero la que tú has utilizado es la más común, al menos es la que yo utilizaría y está perfecta ^-^ espero que te sirva de ayuda ;D

That's not an easy question, they are very similar and they are used in the same way. "Preterito imperfecto" is when someone WAS doing something(Yo saltaba, Tú corrías, Él lavaba...) and what you call "estaba haciendo" form doesn't really exist, It's called "perífrasis verbal", for example " Estaba" + a verb in gerund is a "Perífrasis" ( Yo estaba haciendo la cena, Tú estabas cantando una canción, etc), and the Spanish language has a lot of "Perífrasis verbales". Anyway on the " Estaba haciendo" form, It means "The subject + estaba haciendo + the thing that He was doing". In the 90% of the cases you can use both and the other 10% is not important, I mean, every one will understand what you're trying to say because there are some cases where "estaba haciendo" + the thing you were doing sounds better and vice versa. I'm not sure if you could understand what I'm trying to say because It's very hard to explain the difference between these two words, sorry if you couldn't understand, ask me if you need more help Edit: You should use the estaba haciendo form for saying things that you did, but not verbs. For example, "Estaba haciendo la compra" is ok, but "Estaba haciendo compra" or "Estaba haciendo compraba", etc is wrong

Those two phrases DON'T mean the same, they are both incorrect xD. "Voy en mi camino" means " I am moving (walking, driving, running,etc...) on my road" and "Estoy en mi camino" means "I'm on my road" as if you were staying there. You don't need to use "my", I mean, in English You have to say "I'm on my way" but in Spanish We don't need to use It because the verb gives us all the information that We need, Furthermore, It means that you own a road(If you use "mi"). "Voy" means "I am going". By the way, It sounds more natural to say "Voy de camino" instead of "Voy en camino", You can use both but the first one is more usual.

Soy español y apenas lo entiendo haha (se utilizan muchas palabras de argentina), la canción no tiene mucho sentido al menos para un español. Y sí, papa es como llaman a la patata en Suramérica, y tallaba es del verbo tallar, según parece quien "tallaba" es la Milonga por ello que no le vea mucho sentido, la papa no tiene relación con la milonga. PD: No estoy 100% seguro ya que es muy rara la canción -.-"