Hi Nomad,
This Marlin Papoose is the only semi-auto long-gun that I own, so I am new to the "spitting out debris from the action cycling" problem.
I shoot right-handed and wear perscription shooting glasses while at the range.
On my last trip with the Papoose I found that my persrciption glasses were interfering with my focus on the open iron sights of the rifle (I usually shoot with a scope) so I lowered my glasses for a 5 round group (I know, my bad). I did detect debris being ejected from the action with the spent case and since I was shooting in a very strong wind some of this material was being blown back into my face/eyes.
So when I was shooting after that I would trade my perscription glasses with my daughter for her clear safety glasses and I beat both the focusing and debris problem.
In summary I do not think the Papoose ejected any more debris than normal, just the windy conditions of the day made it a problem.
How this would effect a left-handed shooter I do not know, if I take the rifle hunting tomorrow (opening day for grouse where I live) I will rattle off a clip from the left shoulder and let you know.
Good Luck,
Mike