CarolC-have you encountered any Israeli leaders whose role was similar to Al-Husseini's?

No. I haven't run across any myself. I am not aware of any of the leaders of the Zionist movement or any Israeli leaders who were appointed by some outside political entity as al Husseini was by the British.

Did the Israelis have a say in whether Irgun massacred Arabs?

Well, I think it would be fair to say that the leaders of Israel had some say in what the Irgun did, being the architects of the agenda that the Irgun were fighting for, even though officially they try to maintain an appearance of separation. Whereas, it was the British who put al Husseini in power against the wishes of the moderate Arab majority, and with an agenda that was not agreed to by the majority of the Arabs in the area, or even by the Arab leaders who preceded Britain's appointment of al Husseini to be their new leader.

So, should all of the European Jewish settlers be held responsible for what the Irgun did? No, definitely not. Should the Israeli (Zionist) leadership of that time be held responsible for the expulsion and terrorization of the Palestinians? Absolutely.