They could not find (ouch hēuriskon). Imperfect active. They kept on not finding.
What they might do (to ti poiēsōsin). First aorist active deliberative subjunctive in a direct question retained in the indirect. Note the article to (neuter accusative) with the question.
Hung upon him (exekremeto autou). Imperfect middle of ekkremamai, an old verb (mi form) to hang from, here only in the N.T. The form is an omega form from ekkremomai, a constant tendency to the omega form in the Koiné. It pictures the whole nation (save the leaders in Luk 19:47) hanging upon the words of Jesus as if in suspense in mid-air, rapt attention that angered these same leaders. Tyndale renders it “stuck by him.”