18s 30fr clip will only render 17:29s?

The reason I even noticed anything is because Photoshop limits the import to 500 frames.

are you sure you need frame animation? Photoshop's Video timeline does not have a duration limitation in my experience, and it will allow you to create GIF's. see here for a discussion about it in the end of the thread: Re: Black grain when exporting .gif

BTW if all you need is GIFs, you can use this handy script: GifGun - aescripts + aeplugins - aescripts.com

as for Frame Animation - you can bypass this 500 frames limitation, although this process will be very CPU intensive and maybe not that productive (Ps even mentions this as you import to let you know that even 500 frames is a lot). as for a GIF's duration - make sure you know what you are doing because GIFs usually are not supposed to be this long, unless you are using hold frame method. (see the thread I posted before on what Rick suggests for hold frames workflow)

here's how to have a Frame Animation longer than 500 frames. basically don't import your video with import -> frames to layers, but instead -

Method #1

1. drag you video to photoshop - it will create a Video Timeline.
2. click on the flyout menu of the timeline panel and choose convert frames->flatten frames into clips
3. delete the video group that contained your video. you only need the frames
4. click on the flyout menu of the timeline panel and choose convert frames->convert to frame animation
5. don't worry about the warning - click o.k
6. click on the flyout menu of the timeline panel and choose make frames from layers
7. click on the flyout menu of the timeline panel and choose "select all frames"
8. set your frames to the proper duration.

Method #2

alternatively, you could render you video as a sequence and import it to photoshop as layers using file->scripts->load files into stack

and choose the sequence folder. this will take a while so be patient. at the end you will have a psd file that contains all the frames as layers.

1. open your timeline and choose "Create Frame Animation"

2. click on the flyout menu of the timeline panel and choose "create frames from layers"

3. click on the flyout menu of the timeline panel and choose "reverse layers"

Done

This is standard in all video applications. The time indicator and the number of frames always stops on the last visible frame. That frame still has to play so the time has not stopped until you move to the end of the last frame, which means there is no frame visible in the comp panel or the video.

Does that make sense? Most folks don't really get it until they look at time properly.

Also, it concerns me that your comp settings are Custom. Unless you know exactly what you are doing and you are expert in formats, video standards and frame sizes you should not be customizing any of your comp settings. The comp size you have set will not render properly using h.264 standards. It's just the wrong size. The compression strategy requires specific multiples of blocks of 16 pixels to work properly and efficiently. Check out the standards here before you start creating custom settings.