So just to be clear on what you are saying.
If we can agree that 8 screws will have plenty of shear strength to hold the cupboard and contents.
So hypothetically, when facing that wall of cabinets, the refer. is on the left and the first end panel is against an upper and a base cabinet, which to keep that refer end panel square need to have an 1/8", or an FEP installed to bring the back side of both cabinets level with those cabinets face frame, so the right refer end panel will be square.
The same 1/8" or FEP needs to be installed on the refer cab. to keep it square in the opening between the two end panels.
There is a 6" overlap between the refer cabinet and the wall cabinet on the right, giving you about 1-1/2" of meat to screw into, 5-1/4" of which is probably particle board.
While using flush nuts is a good Idea, the time , fostner bit and accuracy may be beyond some homeowner's wifes patience level.
Are you saying that the wood would fail and the screw would scrape a track so that the cupboard would slide down.
"What can go wrong will", and a wood screw with 1/2" penetration is a failure waiting for an opportunity.
Wouldn't that bring into question those little pins on the same cupboard installed to hold the weight of the shelf and it's contents.
I have seen shelves fail, I have never seen an enlarged hole
They are a tight fit, but, require the user to make up their ---- mind.