 Here is a Python script that uses the moviepy library to perform the tasks as described:

```python
import os
import subprocess
import moviepy.editor as mpy

def download_video(url):
    try:
        subprocess.run(["youtube-dl", "-o", "output.mp4", url])
        return True
    except Exception as e:
        print(f"Error downloading video: {e}")
        return False

def load_and_process_audio(audio_file):
    audio = mpy.AudioFileClip(audio_file)
    period = audio.duration
    audio_with_fade = audio.subclip(0, period).audio_fadein(in_length=1, out_length=1)
    return audio_with_fade, period

def load_and_process_video(video_file):
    video = mpy.VideoFileClip(video_file)
    loop_segment = video.subclip(0, video.duration // 2)
    slowed_loop_segment = loop_segment.set_duration(loop_segment.duration * 2)
    mirrored_loop_segment = slowed_loop_segment.fl_mirror()
    return slowed_loop_segment, mirrored_loop_segment

def combine_videos(slowed_loop_segment, mirrored_loop_segment):
    combined_video = mpy.concatenate([slowed_loop_segment, mirrored_loop_segment])
    combined_video = combined_video.set_duration(slowed_loop_segment.duration * 2)
    combined_video = combined_video.set_audio(slowed_loop_segment.audio)
    combined_video = combined_video.fadein(1).crossfadein(1).fadeout(1).crossfadeout(1)
    return combined_video

def create_title_and_credits_screens(title_text, credits_text):
    title_clip = mpy.TextClip(title_text, fontsize=48, color="white", background=mpy.ColorMap("black", "white"))
    title_clip = title_clip.set_duration(slowed_loop_segment.duration)

    credits_clip = mpy.TextClip(credits_text, fontsize=24, color="white", background=mpy.ColorMap("black", "white"))
    credits_clip = credits_clip.set_duration(slowed_loop_segment.duration)
    credits_clip = mpy.CompositeVideoClip([credits_clip, mpy.VideoClip(mpy.ColorMap("black"))])

    return title_clip, credits_clip

def save_final_video(final_clip, output_file, fps=30, audio_bitrate="192k", video_bitrate="12000"):
    final_clip.write_videofile(output_file, fps_start=1, fps=fps, audio_bitrate=audio_bitrate, video_bitrate=video_bitrate)

if __name__ == "__main__":
    video_url = "https://www.youtube.com/watch?v=dQw4w9WgXcQ"  # Replace with your video URL
    audio_file = "audio.mp3"  # Replace with your audio file path
    title_text = "My Title"
    credits_text = "My Credits"

    if not os.path.exists(video_url.split("/")[-1]):
        if not download_video(video_url):
            exit(1)

    audio_with_fade, period = load_and_process_audio(audio_file)
    video_file = video_url.split("/")[-1]
    video, _ = load_and_process_video(video_file)
    slowed_loop_segment, mirrored_loop_segment = combine_videos(video, mirrored_loop_segment)
    title_clip, credits_clip = create_title_and_credits_screens(title_text, credits_text)
    final_clip = mpy.concatenate([title_clip, slowed_loop_segment, credits_clip])
    save_final_video(final_clip, "output.mp4")
```

This script assumes that you have the `youtube-dl` command-line tool installed. Make sure to replace the `video_url` and `audio_file` variables with the paths to your desired video and audio files. Also, adjust the title and credits text as needed.