Short answer: The action shown in the photo is legal, and the robot is not Possessing more than one cone at a time.
The robot is also not hoarding, because hoarding refers to “intentionally plowing multiple Cones to a specific location of the field, even with a flat/convex portion of a Robot, such that they are kept away from the opposing Alliance.” (See SG9 Note.)
Here are the rules I used (emphasis mine):
SG9 Note 2, the reason why the robot’s action is legal:
SG9 Note (unnumbered), about hoarding:
The top 4 cones (the one the robot is touching and the 3 above that cone) would not count for points at the end of the match, but the other 10 cones would count. (See Note 2 of the definition of Stacked, and the second-to-last paragraph of this Q&A answer.)
If the robot physically lifted the top four cones off of the Stack, that would be Possession of more than one cone, and would be illegal.
Yes, because they still meet all the other requirements for being scored, except a robot is touching them. This would not violate your 1 cone rule, but any cone you touch, as well as any cone above it, do not count as scored