to kick things let me say a big thanks for your tool, i've used it few times but it has some issues(?).
I've used it to download a YT live while it was still playing, first time worked perfectly and it didnt gave me problems when the live got copyrighted(from music) it kept download the rest of it.
Video was complete and everything was good.
Second time i tried using it w/o closing the same app instance as before it wasnt sending any network packet, like it had unclosed connection from first time and no error handling?resulting to "select with timeout Error time limit expired!"<gif attached>
Gave it another chance and tried do the same thing, download a live YT stream which possibly would get copyrighted, the video didnt got any copyright but the download finished 6-8 hours later(i got terrible internet but not that much).
Trying again the exact same thing, monitoring the network and the filesize, ffmpeg had 2.4gb of network total receive while the filesize was ~300mb(weird) which i interuptted downloading.
By opening this video with different media players, none of them could start rendering anything and i wanted to use that ~300mb part since eventually got copyrighted.
about ur questions:
Part 2 audio and video are they the same time?Have u tried to extract audio and video in case they are unsync? Did u try to merge these 2 parts and see if the output works?
i would: copy /b 001-050.mp4 + 051-100.mp4 output.mp4(u can merge as much segments u want) and check or/and u can try this "kernel video repair" which seemed to created an output(not sure if worked) for one corrupted downloads of mine, unfortunately they ask for license even to try it so... "module": "licenseactivation.dll" <they load it dynamically on activation> "address": "0x38B4" i think is the last part u will look at(dont know if i can share such infos public here).
Then you can download it in parts as 001-002 for the time u want and merge them later BUT the problem of using the YT API is that it doesnt split permissions, what i mean its 1 auth to retrieve the information and manage the full channel with uploads and downloads.
There was a service that got shutdown which was giving the time of videos and the oembed from yt doesnt return times(ex: https://www.youtube.com/oembed?url=YOUR_YT_LINK&format=json)
Hopefully i understood correctly and since youtube-dl works fine with non-live my blind guess is u ask for YT LIVES but even if not, someone else might find that information useful.
About the 403 and the cookie i have no clue, sorry.
this message became like an essay wow cheers