Dr. Byte,

It did indeed solve the problem to go to the specific product page. Above the Qty Min, Qty Max, Qty Units boxes there is a choice:
Products Quantity Box Shows
with radio buttons for "yes" and "no." Mine had "no" checked, with a statement in red:

Warning: Does not show quantity box, default to quantity 1
I changed that to yes, and now the changeable quantity box does show on my shopping cart page.

On my Product Info page this was already showing. Above the Add to Cart quantity box already showing, the above change added the statement: "Quantity in cart: 1." So this takes care of the possibility of erroneously adding a second product after navigating away from the shopping cart.

It makes sense that this setting was somehow placed in my cart by Easy Populate Advanced 3.0.3, since that program had also changed the minimum, maximum, etc., settings, and that was fixed by adding columns, so I am sure I can add a column to say yes to showing the quantity box on upload.

Now on to identifying that column.

Thank you very much.

Notageek