Yes, the bit that makes an Intelligence check is the "I am your Master" dialogue option, which doesn't even appear unless you're at or close to the Intelligence level required. The "I seem to have killed your Master" option doesn't have any skill listed next to it, but when Arabella refuses to accept this explanation (because her master, Elithran, is unkillable), her dialogue is preceded by "(Failed)". This makes me think that there is, or should be, a better result possible. I've tried animating the golem that I... uh, inherited from Elithran in the hopes that somehow this would prove I'd killed him, but it didn't make any difference. I don't really think it should have, because I don't get the impression that Arabella knew Elithran was buying a golem.
I'm probably strong enough to kill Arabella if I try, and I can make her believe I'm Elithran and she'll go off in a sulk, but I feel bad for lying to her, and it feels kind of wrong to kill something that's just waiting for its master to come home.
It's realistic for Arabella to refuse to believe me, but if it's not skill-checked or checked in some other way, then it shouldn't be marked "Failed" in the dialogue. Alternatively, there could be a small trinket that you take from Elithran's corpse that she would recognise, either with or without a skill check; this could cause her to attack you in revenge, or alternatively she might go off into a corner and grieve. I don't think she'd suddenly turn helpful, because from the rest of the dialogue she is loyal to Elithran.
I think the "I am your Master" option still has to lead to the best solution for the player, or one at least as good as successfully convincing Arabella that Elithran is dead, because it requires a skill check that's relatively high for this point in the game.
I'll be interested to see what this turns into! Thanks for getting back to me - now I can stop racking my brains for the answer.