If I recall Abbas was elected to one term in office. I don’t recall if it was four years or five, but it was at least ten years ago. The state that elected him technically no longer exists. He has no followers other than those employees he pays with foreign money. They will be loyal to him as long as he has access to other people’s money, but he is not an elected leader. He’s necessary to continue the fiction that there is somebody to deal with. But he can’t make a deal because if he does nobody will follow it. So, when a deal is close, he queers it, as he understands the problem. His goal is to keep the foreign cash flowing so he can suck it off into his personal account, like virtually every other middle east leader.
You’re right. His elected term expired years ago, but he has continued to serve.
The ugly secret of these negotiations is that the PLO is the legal representative of the palestinian people in the eyes of the UN, and short of amending their charter, the deals on the table are illegal. And as you point out Abbas is an illegitimate leader, in office only because he's canceled elections. And 40% of the population is governed by a terrorist organization. Should the Arabs abrogate any agreement reached, they have grounds.