> (1) investigation is not the same as conviction, and

No.

> (2) that repeating the phrase "crooked Hillary" ceaselessly is a marketing tactic

Yes.

> designed to encourage a belief that Clinton is a criminal

No, it brings to light the fact that she was under investigation and has many shady dealings in the past. It is to raise doubt. To be a criminal she must be convicted.

> in order to undermine her legitimacy as a candidate?

Partly. I think more so to raise doubts about what she would be like in power.