Rick,
Your first explanaton as to how this interaction works seems to be the main one. see

It turns out that grapefruit juice can directly or indirectly interact in important ways with a number of medications. This is especially important since grapefruit juice is consumed by approximately one fifth of Americans for breakfast - a time of the day when medications also are commonly taken.

Grapefruit juice blocks special enzymes in the wall of the small intestine that actually destroys many medications and prevents their absorption into the body. Thus, smaller amounts of the drugs get into the body than are ingested. When the action of this enzyme is blocked, more of the drugs get into the body and the blood levels of these medications increase. This can lead to toxic side effects from the medications.

There are over 1 million Americans with PD and if 20% of them take grapefruit juice, it looks fairly safe. The explanation that it blocks the enzyme that breaks down levodopa in the body, before it can get to the brain, explains why i got a boost. As one of your references said, it's effect is like taking a higher dose of Sinemet.
Has anyone else tried it, and got a boost?
Ron