Kyle Katarn is a very powerful Jedi, so he gets a pass on it. Dash Rendar doesn't do anything too far out of the ordinary for the type of character he's supposed to be. Starkiller on the other hand shouldn't be taken too seriously. Word is that a lot of Starkiller's feats are more or less tall tales told by the Rebels about their founder.
Healing doesn't break the rest of the lore since apparently very few people are good at it, as >>2077
says. This applies to many of the powers in JK outside the basic powers. The JK power that actually breaks the rest of the lore the hardest is Absorb because it renders most of the Sith's favorite powers useless when maxed out. If you're a Sith up against a Jedi who knows Absorb, you might as well just use a flechette shotgun instead of bothering with Lightning. They can still Protect against it, but they can't keep that up indefinitely. Yoda's fight with Dooku shows why Absorb is the king.
>Kyle Katarn almost a second Revan
Revan is far more skilled than Kyle in every respect other than individual battle prowess. Revan is a strategic and tactical wunderkind in both land and space battles as well as building HK-47. Kyle is just very good at fighting. He later becomes more politically adept, but Revan still outclasses him here.
>Dash Rendar somehow beat an AT-ST and IG-88 on his own.
The Imperials put the AT-ST in a location where there was a lot of easily accessible cover and missile pickups for Dash to use, so that was less about Dash being super smuggler extraordinaire and more about Imperial incompetence. Also, the AT-ST was never all that tough to begin with. Several AT-STs were destroyed on Endor by primitives with rock slings. It's not a frontline heavy vehicle, it's a scout vehicle. IG-88 is a badass, but we have plenty of other examples of human troops taking out battle droids and assassin droids.