I know this will be seen as no help at all, but what you're seeing is precisely why I use Premiere for video editing and I'm learning AfterEffects for animations. They're made for that. Photoshop has the (limited) ability to work with animations and video rendering, but it should never be relied upon for that.
You have to provide exact render/ export settings. Many CoDecs have internal framerate controls that can ovveride external control and presumably that's what you are seeing.
If you are reducing the frame rate, PS does the frame rate conversion but does not change the clip's duration. Since the clip's true duration is now longer, click on the right edge of the clip and drag it to the right making the clip longer. This dragging will stop when the full clip is exposed.
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Photoshop CS6 and CC support two types of animation Frame animations and a Video Timeline
Changing the frame rate should not change the length of a video timeline. It should just change the number of frames that are rendered. To change the length of a video you need to change the length of the video timeline. If you increase the length of the timeline and there is audio I'm quite sure there will be a problem with it, The Video part will most likely be better then the sound. The required number of extra frames can be generated however the video may look like slow motion. If you decrease the length the motion will speed up. The length of a video is normally done be editing video clips which are trimmed and stitched together to get the desired duration. Not normally done by altering the duration of a clip.
The duration a of Frame animation is the sum of the times each frame is displayed. In a frame animation the length of time a frame is displayed can be varied it not a normal motion picture video. To have a smooth frame animation you need a sufficient number of frames that are different from each other to show smooth motion. You can use Photoshop Frame animation tween feature to have Photoshop manufacture frames to help with a smoothing the animation. Once you have a frame animation the length with the motion you want you can have Photoshop convert it to a video timeline and render a normal motion picture video. It will not have audio but you can add audio to it.