Anwsers...
1) all plane shift patterns in the Matrix start from Elbow Plane; for golfers who tend to start from Hands Only Plane there's this "+ HOPA" thing; what about the plane shift when a golfer returns to Hands Only Plane at impact ?
if you ask - who ? I answer - e.g. Hogan with short irons or wedges...
There are going to be maybe a few more plane angle shift combinations in the whole "Matrix" lexicon.
The "Hands Only Plane" is only the "Hands Only Plane," if you CAN'T get your elbow socket on this angle in an orthodox impact.
There are a mutitude of golfers who have the SHAFT on the elbow plane at address, then return to this plane angle WITH the elbow in place at impact.
This is possible due to the very differnt BODY location at impact, which should include a lower and more forward Right Shoulder allowing the right arm to be more BENT than at address, and have the elbow, and sometimes the WHOLE FOREARM on the elbow plane at impact.
Here is an example of that, with a very standard double shift—Ben Hogan from his "Power Golf":
2) can a golfer's plane shifts differ depending on a club that he is currently using ?
Absolutely.
I use one pattern for drivers and another for most irons shots. There are many differences INCLUDING—but not exclusively—Plane Shifts.
3) are the plane shifts valid for full swings only ?
I sure wouldn't want to do it on a chip shot, but on lots of pitch swings, there is a shift.
4) isn't a golf stroke engineering a very fascinating stuff ?
I love my job.