What computer operating system is your Premiere Elements 12 running on? Have you updated 12 to 12.1 Update using an opened project's Help Menu/Update?
If not, please do so.
Please clarify "Can't add files filmed in 1080/60i to Premiere Elements 12."
Are you importing the 1080 60i into the project using (Expert workspace) Add Media/Files and Folders/Project Assets?
If so, are you getting an error message inside of the video appearing in Project Assets?
Are you setting the project preset manually or letting the program do that task automatically?
What is the project preset?
(See Edit Menu/Project Settings/General).
The project preset should be, for your 1080i...
What is the project preset for the 1080p footage?
(Also see Edit Menu/Project Settings/General.)
The project preset should be, for your 1080p
DLSR 1080p30 or DLSR firstname.lastname@example.org
Since your camera appears to be producing AVCHD.mov files, do you have the latest version
of QuickTime installed on your computer with Premiere Elements 12/12.1?
The answers will be in the details. Please review and then let us know the details and results
based on the above.
So I need the latest version of Quicktime to use Premiere Elements?
Premiere Elements 12
Have you been running Premiere Elements without QuickTime?
QuickTime is a Premiere Elements (any version) requirement.
If you do not have it installed, note that you do not have any QuickTime presets under
Publish+Share/Computer/QuickTime. Other purposes as well.
Please continue the troubleshooting after you install the latest version of QuickTime on your computer
with Premiere Elements.