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)