If you want offline, don’t chose Plex.

I have Plex premium. When there was an internet outage in my area, Plex struggled to play even offline media.

China (and local business men) usually have multiple times of profit from the same movie.

Early released movies are not good quality. But they would sell it and people would buy it. When the movie was officially released, they would sell the same movie again as better version of the movie. Sometimes they will even sell 4 in 1 (4 movies in a single disc), different movie with same name, prequels and BluRay version.

I think your choppy version is the product of the early profit model of Chinese business men.

Most disc movies over there are counterfeits, not official.

I live in a neighbor country of china. We grow up watching movies on VCD, DVD from China. There is no shop or market for original movie VCD or DVD. The original would be too much expensive and no one will buy.

If you get the software from reliable source, it is almost certain it does not have malware.

The piracy is an organized activity and people at higher rank nuke the release if it has malware.

Read more here https://opentrackers.org/i/2600_Guide_to_Internet_Piracy-TYDJ.txt