I think that your question would be better asked in the Adobe Lightroom 5.4 Forum since the issue is with Lightroom 5.4 and not Premiere Elements.
Please repost your question there or wait for a moderator here to see your thread and move it from here to there.
While you are waiting for all this to happen, I would ask you to think about
a. What module of Lightroom 5.4 are you working in for playback of the Nikon D90 and Nikon D610 videos?
b. Are the properties of the Nikon D90 and Nikon D610 old and new recordings the same?
c. Did you work with both Lightroom 5.3 and 5.4 on this same Mac computer?
I currently have the tryout of Lightroom 5.4 installed on my Windows 8.1 64 bit computer. If I can find a Nikon D90 and/or Nikon D610 sample online, I will give it a look in Lightroom 5.4 tryout before I uninstall the tryout and post the results in your thread wherever its final destination.
I have found some Nikon D90 and Nikon D610 videos online and have downloaded them for testing in Lightroom 5.4 on my Windows 8.1 64 bit computer. I do not have a Mac computer available to me.
It would appear that your Nikon D90 is producing MotionJPEG.avi (1280 x 720 @ 24 progressive frames per second); whereas, the Nikon D610 is producting AVCHD.mov (1920 x 1080 @ 29.97 progressive frames per second).
Are those the properties that you are seeing for the videos with which you are trying to work in Lightroom 5.4? And, are you still having the same playback failures in Lightroom 5.4? Again, please confirm the Lightroom 5.4 module involved.
I had no problems plyaing sample videos from the Nikon D90 and Nikon D610 in Lightroom 5.4 tryout on Windows 8.1 64 bit.
Playback of these videos was possible in the Library as well as the Slideshow module. And, it was possible to create a 1080p slideshow in the Slideshow module (AVCHD.mp4 1920 x 1080 @ 29.97 progressive frames per second).
Sample files were
Two - MotionJPEG.avi from Nikon D90
One - AVCHD.mov from Nikon D610
At his point, I would ask, if the issue is restricted to videos from just these two cameras. Or, does the issue apply to playback of all videos in Lightroom 5.4 Library and/or Slideshow?
Hopefully the Lightroom people will have more insights into the core of your Lightroom 5.4 video playback issue.