```python
from typing import Dict

def trace_relay(links: Dict[str, str], start: str) -> list:
    if not isinstance(links, dict): raise ValueError("links must be a mapping")
    if start not in links: raise ValueError("start is not a post")

    route = [start]
    visited = set()

    while True:
        current = route[-1]
        if current == "":
            break
        next_post = links.get(current)
        if next_post is None or next_post in visited:
            raise ValueError("Watch comes back to a post already visited")
        route.append(next_post)
        visited.add(current)

    return route
```