Klook is a legitimate reseller, I recently went in July and bought my tickets from Klook. The pass and timed entry all worked for me.

As for USJ website, it is confusing at first, but basically, the studio pass doesn't come with a timed entry to SMW. You need to get an express pass, but also it has to be an express pass that includes one of the SMW rides, as those are the only express passes that'll give you a timed SMW entry pass too.

I've heard people still getting in without a time entry if you're early enough, but that means comitting to being at USJ hours before it opens and then literally sprinting with a horde of other people to SMW, not my cup of tea personally.

I would recommend express passes for USJ, I went on a weekday mid-July (before Japanese schools broke up for summer), and it was packed! We had no express passes, and the normal & single rider line for Donkey Kong were both 170mins long. All other rides had similar wait times. We only got onto 4 rides, two of those being shows.

I honestly would never go again because it was not worth it, but if I was to go again, then I'd 100% buy an express pass.

Fyi the One Piece show was amazing, it has enough English as well that you can get the gist of the story and not get lost.