So i downloaded some videos, the some i can't "jump" between portion of videos. (49ekdv00405, 49madm00007) 

The majority of the videos downloaded though, i can't render with any player or codec, even converter app can't read it. (57bdsr00187, 125umd0772, h_910vrtm00059)

Possibly an issue on r18 side, as their streaming player is also extremely slow, but I think i need a confirmation.

The videos downloaded are in TS format, which does not contain time-code information. If you want proper seeking, you need to change the container format to MP4, MKV, or something that has time-codes. TS format is used for streaming (which makes sense online), but is terrible for desktop viewing.

yes but only a few of the videos are "only" unable to jump, the rest are unable to render

Assuming the videos that you downloaded are not corrupted, you can try TS-Doctor (Cypheros). They have a 30 days fully functional application and can play TS files without any problems. It is mainly for satellite recorded videos but will play any TS files.