the only reason Naruto was able to use sage mode was because the kyuubi's chakra mixed with his own granting him with more "natural" chakra hence why he could train to become a sage
no. i don't know where did you get this but totally wrong. because of the kyuubi he can't merge with Ma and Pa, but the kyuubi's chakra doesn't have any business with sage mode.
about Sasuke, yes he get his powers for "free". Susanoo, Amaterasu, Tsukuyomi (or how do you spell it) all given to him by Itachi he didn't even trained for it. MS also awakened by Itachi. EMS given to him by Madara (Itachi's eyes...). I don't know how he awakened his 1 tomoe sharingan, but the 2 tomoe awakened against Haku, the 3 tomoe against Naruto, both of them in battle not by actual training, it's just awakened out of nowhere. curse mark also given to him by Orochimaru, the only thing he had to do is to remain alive. the only thing he had to train is his chidori and it's variets.
some of you are saying for all of these he had to pay. but what? and train. but how? he didn't payed for the curse mark, he only wanted to be alive to kill Itachi. it's not payment, not training, it's nothing. everything Itachi gave him was also free. a gift for his little brother. Itachi was sick and almost dead, but if he decide not to fight with Sasuke he would never get MS and it's power at all. but they fought and Itachi died for Sasuke's sake and gave him his powers. it was a hard training? no. it was a payment? not really. Itachi would be dead either way if they fought or not. it was a gift. yes.
to finish my post, I'm not hating Sasuke however i don't like him at all. almost everything was given to him.
he doesn't explicitly show how difficult something is for him because he's... cool.