No your cloth bags are not the solution. It's a trade off between immediate needs and future costs. I don't dispute plastic bags have many negative externalities however they are profoundly immediately useful to humans. There are massive unintended consequences whenever you impose a designed objective function on reality as a planner.

In regards to the questionable surfaces I don't dispute that either but it is irrelevant.

what the relevant issue is measuring the baseline transmission effectiveness from a outbreak source prior to the behavior change vs after.

It is a physical fact that a shopper, for example a horder, who can provide source bacterial matter has been increased in likelihood to bring a bag with such matter and then when checking out has an exponential increase in contact with others. There are documented cases of people using hospital bags for their groceries.