I'll try and answer these questions immediately, and hopefully will have the beginning of a glossary or such ready later today.
Are we actually somehow obligated to do as he wishes?
Once he gives a 'real' order, we will be obliged to do so, until then, we won't. I'll make the order noticeable enough, though.
If yes, how? Is there a time limit?
Time limit and other terms will be set by the order itself. If it's just "Kill my master", you can take your time. If it's "Kill my master until sunset tomorrow with a yellow elephant" (I should stop writing about yellow elephants), things will get complicated.
You are forced to follow the order due to evil things (tm) happening to you, otherwise. Of the 'destroy your essence completely' variant. Or the GM railroading, maybe.
What happens if he steps into the pentagram? Can we exit it on our own will?
If he steps into it, you can do anything you like to him. If that dismissal rune would've been drawn properly, you'd afterwards be flung back, but at the moment... And no, you can't get out until he allows it.
What happens if we attack him?
At the moment, you can't affect anything outside of that pentagram. That's the direct protection rune.
Afterwards, if he isn't an idiot, he'll include "Do not harm me" in his order. Otherwise, you can do anything you want, as long as you still follow his orders.
If he gives us two commands in a row before the first one has been completed, which of those are we obligated to follow?
You simply follow the first 'real' command he gives you.