Well, unless you have a monitor the size of a battleship you'll never be able to view 100% of the image at 100% (not at the dimensions specified anyway), so if you CAN see the whole image then its obviously NOT being viewed @ 100%.
You cannot use this to ascertain by how many pixels something is out.
If it "looks", NOTE, "looks" like its 1px out when it fits the workspace then obviously when viewed at 100% its going to be far more.....your screen pixels don't change size when zooming so yeah, its normal.
Well, thats this bit.....the 'X' is the transform handles....automatically enabled if your settings tell it to be...
As I said, I cannot replicate the problem, I've tried it all ways and have relayed the results to you.
One thing I forgot to mention is make sure when you 'Place' an image check the Width and Height %, (top of workspace)...
[ATTACH]46651[/ATTACH]
If this ISN'T 100% for both then the incoming image is NOT the same size as the document.
I can only assume that either the original image or the 'placed' image are not the same size......are you sure your original image is not bigger than the canvas?
You can check this by selecting the layer its on, use Ctrl + T, (to get the afore mention 'transform' handles)...
[OPTIONAL]:Nav to VIEW > 100% (or VIEW > 'Actual Pixels' on older PS versions) and check for edge alignment.
Then check the INFO panel for the dimensions of the TRANSFORM.
It should match the doc dimensions and in your case be 7990x7750px.
[ATTACH]46650[/ATTACH]
Here you can see that my image is actually bigger than the canvas (document, ( Doc: )).
If you get something similar to this:
[ATTACH]46649[/ATTACH]
...Then your image is bigger than the canvas and you should prepare for a stay in the dog-house.
Save from attaching the files in question so we can look at it directly I really dunno what else to suggest.
Regards.
MrTom.