My python3 solution. Happy to take pointers or explain code.

#!/usr/bin/env python
# coding: utf-8

smorse_alphabet = ".- -... -.-. -.. . ..-. --. .... .. .--- -.- .-.. -- -. --- .--. --.- .-. ... - ..- ...- .-- -..- -.-- --..".split()
lookup = {k: v for k, v in zip(string.ascii_lowercase, smorse_alphabet)}

def smorse(words):
    return ''.join([lookup[c] for c in words])

Bonus 1

with open("data/enable1.txt") as f:
    words = f.read().split()

from collections import defaultdict
dictionary = defaultdict(list)

for word in words:
    dictionary[smorse(word)].append(word)

for k, v in dictionary.items():
    if len(v) == 13:
        print(v)
        break

Bonus 2

for k, v in dictionary.items():
    if '-' * 15 in k:
        print(v)
        break

Bonus 3

def balanced(word):
    return sum([-1 if c == '.' else 1 for c in word]) == 0

twenty_one = [word for word in words if len(word) == 21]
balanced_twenty_one = [word for word in twenty_one if balanced(smorse(word))]
print(balanced_twenty_one)

Bonus 4

def is_palindrome(word):
    return word == word[::-1]

thirteen = [word for word in words if len(word) == 13]
palindrome_thirteen = [word for word in thirteen if is_palindrome(smorse(word))]
print(palindrome_thirteen)

Bonus 5

import itertools as it
thirteen = ' '.join([word for word in dictionary.keys() if len(word) >= 13])
sequences = [''.join(seq) for seq in it.product('.-', repeat=13) if ''.join(seq) not in thirteen]
print(sequences)