Support fluctuates, and in my experience, both Israeli and Palestinian public opinion, support, voting polls are quite susceptible to the effects of current events. There's a quite common observation with regard to Israeli politics, which IMO applies to the Palestinian side as well - namely, that voters might be unsatisfied with and critical of Netanyahu, but still vote the same on elections. Reason being the vote is less rational, and more effected by emotions, group affiliation, election trail rhetoric or simply voting against the other team. I often reference the resemblance between Netanyahu and Trump - in this case, think Republican voters.
I'm not exactly sure which 'other' parties/options make up these 15%, but as I understand it the options include specific regional candidates (not so good for Fatah), or anti (government, corruption, economy) parties indicative of public dissatisfaction. So while maybe not directly affiliated or supportive of Hamas, IMO they do take away votes from the Fatah.
In addition to all that, what you refer to are general support figures. While you may disregard how elections work, that would be a mistake. If the same election system is applied, Hamas could still come on top - maybe not as decisively, maybe having to form a coalition, but anyway a force in domestic politics. To highlight this - the general vote count on the recent Israeli elections saw the current coalition win by a small margin of several ten thousand votes. Translated to parliament seats, however, it resulted in a 'stronger' majority.
As for what may or may not happen, we'll obviously have to wait and see. I think it quite probable that Israel would further curtail Hamas's political activity in the West Bank after the war, so some of the discussion above may prove irrelevant.