What limits do two incorporeal creatures have when combating one another?
"Incorporeal creatures take full damage from other incorporeal creatures and effects, as well as all force effects."
What limits does an incorporeal creature face when attacking a non-corporeal creature?
Incorporeal is the same as non-corporeal. It is lacking a corporeal (real, physical) body.
If you mean an incorporeal attacking a corporeal (or non-INcorporeal) creatures, then it can't make physical attacks (because it has no body) but it can usually make some kind of incorporeal touch attack that ignores non-force armor and shields.
Would non-force spells be at half-effectiveness?
Not sure.
Blink defines Etherial as "invisible, incorporeal, and capable of moving in any direction" and says "spells you cast while ethereal affect only other ethereal things."
However, Etherial Jaunt doesn't mention incorporeality, only that you are "invisible, insubstantial, and capable of moving in any direction"
Perhaps Etherial is a more advanced version of incorporeal. So, while an Etherial creature can't Fireball a goblin, an Incorporeal one could? Since Incorporeal doesn't mention these restrictions, I think that's the intended way to go. Otherwise it would be very strange to have things like a Ghaele Azata, which can use spell-like abilities while incorporeal (via Light Form).