- The walking thing you can mostly calculate yourself if you only examine particular foods, but here's some study looking at the dietary impact in practice averaged across a country (walking has comparable emissions to a 22mpg car trip in developed countries) [0].
- For cotton reusable bags (very common for all sorts of reasons; all my reusable bags are cotton and not because of any particular intentional selection), you need 50+ trips to the store to hit a break-even point [1] in greenhouse emissions. Similarly with the 50x thicker plastic bags stores in CA sell compared to disposable shopping bags. That's 1-2yrs for the break-even point with weekly or biweekly shopping trips, worse if your usage distribution is temporally nonuniform (e.g., owning 5 bags but sometimes only using only 1-3 for slightly more frequent shopping trips and occasionally using all 5). Properly reusable bags are likely worth it, but it's not an immediate or obvious win unless you use them regularly and they're sturdy enough to not wear out too quickly (enough material is involved and the timescales are long enough that you should also consider the impact of the disposal method and a number of other things).
- Some of the other points like linearity in the number of days between shopping trips should be obvious. I'll leave investigating everything else as an exercise for the reader.
You are making the baseline assumption that the reason for reusable bags is to reduce greenhouse emissions, instead of reducing the number of plastic bags being discarded in public areas causing negative externalities like affecting wildlife. Heavier plastic bags have a greater chance of lasting beyond a single shop trip to make it long enough to be reused at the very least as a garbage bag. Paper bags desintegrate easily if thrown in the trash and are trivially recyclable.
I do think that we need to meet people where they are at, you can't expect people to over night start bringing a classic trolley and/or canvas bags to a local shop if they don't have a shop to walk to or dont have a way to keep a bag on themselves at all times, but we can slowly nudge people towards desirable behavior. And that's what charging for bags does.
