You can visit the Krungsri cineplex website to check out the showtimes for the Paragon IMAX hall. You can find their website by searching/googling "major cineplex thailand" By default language of the website is Thai, but you can choose English by clicking the En button on the top right side of the main webpage. On the drop down list of "Selected Branches", select Bangkok:Urban -> Krungsri IMAX Paragon Cineplex, then click Showtimes. The Hall number is IMAX 1. From there, it's quite easy to choose the dates you're interested in, and the showtimes. There are options to either Reserve or Purchase the tickets. So far, I have only used the Reserve option. For Reserve, only certain seats are available for selection, marked R on the hall seating plan. So far, I haven't used the Purchase option, as it'd proceed to some online credit payment page, in Thai language, which I am not proficient in. Considering that the space or legroom between the row of seats are very narrow, I recommend that you choose Row A as much as possible, so that when you sit down on the seat, and lean back, it wouldn't lean onto another person's legs. I hope your online reservation process goes successfully.

Only in peak times. Eg Friday night and Saturday nights.