Merlin shrugged because it was rare that he had visions of himself, but less rare that he beat himself up over his inability to actually change what he saw; he did feel guilty for the bad things that happened that he couldn't do anything about.
Zach's suggestion was possibly more frightening than being pinned against the counter and kissed. Date? A date, didn't Zach just hear what Merlin had said, Zach wouldn't want it and he wouldn't want it. It was probably the worst thing he could think of doing but his heart overrode his head and what he said was, "Yes, I'm going to be free on Sunday."