Hang on lets keep FCPX out of the equation for a moment.
You say you have a 'test' clp from Lens Rentals. In what form is this?
Is it a BPAV folder with with a heap of other folders nested in it?
or
A XXX_xxx_000.mov XDCAM EX 720/1080 etc single movie file?
If its a BPAV, use XDCAM Transfer (v2.12.0) to 'unwrap' the .mp4 file and place it on your desktop as a destination.
Open it with something like QT or Mpegstream Clip, hit cmd i and check the stream info window.
should be something like:
Stream: BAE_HYBRID_0016_01.mov
Path: /Volumes/MyBook/••PROJECTS/B4B_81_12/HEV VAULT/BAE_HYBRID_0016_01.mov
Duration: 0:00:08
Data Size: 36.76 MB
Bit Rate: 36.36 Mbps
Video Tracks:
XDCAM EX 720p50 (35 Mb/s VBR), 1280 × 720, 50 fps, 34.83 Mbps
Audio Tracks:
(lpcm), 768 kbps
Stream Files:
BAE_HYBRID_0016_01.mov (36.76 MB)
...does the file play?
If its already an 'unwrapped' .mov check that in QT or Mpegstream Clip
Do you have much the same in the stream info window? Does it play?
If the answer is yes to both:
go througn the usual 'Import File' for FCPX.
Does it play?
If not then something wrong deep in FCPX and not the 'QuickTime engine maybe.
Sorry to be so pendantic but its good to eliminate the varibles.
good luck.
btw another great little utility for getting into BPAV .mp4s is ElegatoTurbo HD.