More news...
...And I may have proven my case.
Thanks to Ken's hint, I did manage to load the file created with X2 into the VS10 environment. From there, I performed all the normal steps and guess what... I burned this file to a DVD with Ulead VideoStudio 10!!!
The facts:
-- This computer runs on Windows XP Pro
-- The DVD drive is the same (it was swapped from my other laptop)
-- The disc is of the same type as the ones tried with X2 (DVD+R 4.7 GB) in the other machine.
Where the hell does that "Power calibration" message come from? Where is the culprit?
Now this comes to remove a range of the previously suspected (or suggested) sources of the problem:
-- It does not come from the Matshita DVD drive...
-- It does not come from the discs...
So where can it come from?
Fact in point: It may still come from X2!!! Or it may still come from a bad work relationship between X2 and Win7...

Unfortunately, I installed VS10 to my Windows 7 machine but, as I suspected, there are compatibility issues between the two, so I could not complete this extra test.
Here's what happened:
I could load, edit and trim my video clip captured originally with X2 but, for reasons unbeknown to me, when I click the "Create disc" button, it leads me, as expected, on the screen that reads "Add media" -- "Add/Edit chapter" -- "Create menu"... The clip even shows there in its expected place, except that, at the very bottom of this screen, the dark-blue bar with those three buttons for the settings on left side and (even more importantly) the "Next" button on the right side are all missing... And this, of course, prevents me from advancing to the next step (menu creation and burning).
If I could have this last test completed, I would have the final proof to enable me to say, without any doubt, that the fault resides squarely on X2, as I more and more come to suspect...
In any case, though, do I have already a sufficient basis for presenting my case to Corel?
I believe I do! What do you people think?
To all of you, I must reiterate, your hints and your continued help have been the one single reason why I managed to come to this point. I hope they will still lead me to a clearer, more definitive conclusion.
Many thanks.