There really wasnt much that Democrats could do. Mitch controlled the agenda in the Senate at the time (and his friend Lindsey controlled the agenda in the Judiciary Committee). Every prior nominee, even the ones who were controversial, still got hearings and a vote, even if that vote failed. (The one exception might have been the absolute moron that GWB nominated, who was so clueless she failed the written questions* that the committee gave her, but I think she backed out after that so it never got a chance to progress).
The problem with Garland is that he was the compromise candidate. If Mitch let his nomination go to a vote, it would have passed. So he simply ignored it. The only person who might have been able to get around it was Graham, if he had progressed the nomination out of committee, but he didn’t.
The Senate doesn’t have any avenues to force a vote if the Leadership doesn’t want that vote to happen.
In mid-October, the Senate Judiciary Committee requested Miers resubmit her judicial questionnaire after members complained her answers were “inadequate,” “insufficient,” and “insulting.”