Did you not read the scan above? The manga disagree's with you.
Right now you're just being a plain idiot and taking the scans literally.
It's kind of like this.
Imagine you are one buff dude, master of 15 different martial arts, agile, smart, weapon master and you have great reflexes and you are in expert in parkour.
You say ''I can't kill him'' ''I can't kill my mother''
In fact, you can kill.
It will just have a side effect, for example you will be wanted, chased by the police, you can also experience emotional damage.
And then you have
for example, Superman Prime One Million, or just regular Superman.
''I can't be penetrated by regular needles''
That's because you just plain can't do it.
That's what you think is Obito's case, but you're wrong.
But in Juubito's case it's example nr.1, it can happen, only it will have side effects.
He can't faze otherwise it would most likely let the Juubi escape or something in that direction.
Now, let's say it again.
Obito is able to normally create a hole between dimensions allowing him to travel through dimensions, but because he has a monster inside him doing so isn't smart, but he can still do it.
So, if Kakashi created said S/T hole between dimensions said person can still jump through it, anyone can, or he can just put an arm in and close it off.
Basically what Kamui does is create a gate in a certain spot and take away the target to another dimension.
So, Kakashi can still Kamui Madara/Obito juubi jin, it's not like you create a interdimensional gate and because you have some kind of a monster in you that isn't even protected by fabled divine law that you can't jump through it or let a certain body part faze through dimensions, you can do it.
It's just not the optimal choice unless you want to lose it.
