I've just started setting this up for my own family with plex instead of jellyfin, so I don't have a LOT of answers, but...
- yes pinchflat allows you to define the date at which it starts downloading. For a couple channels, I set it to only download the past year's worth of videos and it seems to have respected that properly. It also allows you to set a retention period
- it allows you to download, embed, and use autogenerated subtitles (three separate options)
- Can it limit the time range of video to download? Some channels may have ten thousand of video.
- Can it auto include the CC to video, that's one of main selling points of youtube to me.