```python
from typing import List

def pack_entries(entries: List) -> str:
    if not isinstance(entries, list):
        raise ValueError("Input must be a list")
    
    def escape_value(value: str) -> str:
        return value.replace("\\", "\\\\").replace("=", "\\=").replace(";", "\\;")
    
    def escape_key(key: str) -> str:
        return key.replace("\\", "\\\\").replace("=", "\\=").replace(";", "\\;")
    
    def serialize_pair(key: str, value: str) -> str:
        return f"{escape_key(key)}={escape_value(value)}"
    
    def serialize_entries(entries: List) -> str:
        return "; ".join(serialize_pair(key, value) for key, value in entries)
    
    if not entries:
        return ""
    
    if not all(isinstance(entry, list) and len(entry) == 2 for entry in entries):
        raise ValueError("Each entry must be a two-element list")
    
    if not all(isinstance(item, str) for item in entries):
        raise ValueError("Each key and value must be strings")
    
    if any(key == "" for key in entries):
        raise ValueError("Key cannot be empty")
    
    if any(key == value for key, value in entries):
        raise ValueError("Key cannot repeat within the input")
    
    return serialize_entries(entries)
```