```python
from typing import Dict

def trace_relay(links: dict, start: str) -> list:
    """
    Trace a sequence of posts the watch is handed from in order from start to the post that hands on to the empty name.

    :param links: A dictionary mapping each post's name to the name of the next post.
    :param start: The starting post name.
    :return: A list of post names visited, starting with start and ending with the final handoff post.
    :raises ValueError: If the links argument is not a dictionary, or if start is not in links,
                         or if there's an invalid handoff between posts, or if a loop is detected.
    """
    if not isinstance(links, dict):
        raise ValueError("links must be a dictionary")
    if start not in links:
        raise ValueError(f"start '{start}' is not in the links dictionary")

    visited = set()
    current_post = start
    result = [current_post]

    while current_post != "":
        if current_post in visited:
            raise ValueError("Handoff loop detected")
        visited.add(current_post)
        if current_post not in links:
            raise ValueError(f"Invalid handoff: {current_post} does not have a next post")
        current_post = links[current_post]
        result.append(current_post)

    return result
```