My personal experience was that only British Airways checked for a return ticket, which they insisted I buy, and never the Israeli Border Guards.
I suggest you purchase two one-way tickets. To Israel, at the best price you can find. And a return from Israel at an expensive, fully-refundable fare. When you get to Israel, cash it in for the refund. Then get a cheap one-way at the end of your stay.
However, you may be able to do even better. Your program sounds like it requires a visa, since the standard tourist visa is for less than one year. It is much easier to get this in advance, even if it can be obtained in Israel. And with the visa, the airline will not care about the return ticket, even British Airways.
Caveat: As part of some clever profit maximization plan, British Airways charged me about GBP 1400 for the refundable ticket, and tried to refund it for USD 1400. Numerous complaints to the bank were necessary, and a letter to BA Customer Service saying if they attempted to charge me again, they would hear from my lawyer. I suppose I should have said solicitor.