I think that I have just found a sample GoPro 720p120 that I can download and experiment with in Premiere Elements 11 Windows. The file is downloading now.
If the download is successful, I will do some exploration and experimentation of the file in Premiere Elements 11 Windows and give you my comments on the experience.
This is what I would like to try in your Premiere Elements 11 Mac.
I have been looking over the following Adobe Premiere Pro Forum thread on the subject of that 720p120.
It would appear that both Premiere Elements 11 and Premiere Pro CS6 are recognizing the footage as 59.94 frames per second instead of 119 or 120 frames per second.
Could you try to import your 720p120 into Premiere Elements 11 Mac with each of the following project presets....
NTSC AVCHD AVCHD-LITE 720p60 (that seems to be the one that the program picks out)
NTSC DSLR 720p60 DSLR 720p60.
And let us know the degree of slow motion that you perceive in the Timeline rendered playback of that footage.
I have no experience with 720p120 so I am just experimenting at this point.
Just starting my day and have something for you to try.
Open Premiere Elements 11 Mac, File Menu/New/Project and set the project preset to NTSC DSLR 720p DSLR 720p60.
Then, with the program's Add Media/Files & Folders, import your 720p120 video into the project and drag it from Project Assets to the Timeline (Expert view).
Then Tools/Time Remapping and target the whole clip for Fast Motion with the Forward/Fast/2x setting.
Compare the playback results of rendered
a. Fast Motion Effect video
b. Video to which just Interpret Footage, changing Frame Rate readout from 59.94 (which Premiere Elements is recognizing) to 119 frames per second
c. Video as dragged to Timeline
Next, look at export of Timeline as AVCHD.mp4 file @ 59.94 progressive frames per second. (See Publish+Share/Computer/AVCHD/MP4 -HD 720p60).