Sage-Mode only gives him a constant increase in power, if he has Fukusaku there to constantly gather it for him... otherwise he can gather it to a certain point, and it will be used up in a short ammount of time. Fukusaku said that Sage-Mode would help him recover his Spiritual and Physical energy much like the Kyuubi's power would...
Naruto has the ability to control the Kyuubi's energy, as seen in the last anime, it was the first time we saw him using the Kyuubi eyes, but they were Blue instead of Red... so you're wrong when saying that by using the Kyuubi it harms him and shortens his lifespan... it only harms him when Naruto is unable to control the Kyuubi... During training he was utilitzing the Kyuubi's power... that why when he was overcome by the Kyuubi cloak, he went straight to 3-Tails... and he now has control over the Kyuubi up to 2-Tails... In theory, if he is able to control the Kyuubi up to 5-Tails, and keep his eyes Blue... then he will have complete control over the Kyuubi and ALL of its power...
Don't be mistaken... when Naruto says he doesn't want to use the Kyuubi power... he means that he doesn't want to give into it having control over him... he admitted to letting the Kyuubi take control of him to go into 4-Tails to kill Orochimaru... but Naruto learned that by doing so... he risked the life of Sakura... he won't ever give into the 9-Tailed fox again...
