bentonite vs plastic sheet

Plastic film is inherently weak and easily punctured. Multiple layers do not address the weakness. You will still get punctures. Only now it will allow water and material between the layers where it will decompose. This will create a stench and slipperyness between the layers of plastic.

There are thinner membranes, such as those sold by Home Depot for ponds, as well as the thin roof membranes. These do work, and stand up to mild foot traffic better.

In your application, you would do well to establish the foot paths and support them under the membrane. Pressure treated 2x4's will hold up for years and distribute the load. To a somewhat lesser degree so will pressure treated plywood.

You can keep foot traffic off the ground with raised paths as well. This would allow you to use the cheaper plastic under a protective cover. This may be a viable alternative for a single path in a restricted area, as you seem to describe it being.

I'm not sure what problem you are trying to solve. You cannot posssibly have foot traffic on gravel over plastic. It won't last. They sell a rubber "pond liner" made for that purpose that will seal the drainage area to your drain very nicely. It ain't cheap but it does work. If you want a foot path through the rock garden, I'd just lay a good crushed rock base, build it up and put stepping stones on it. If the path is in the middle of the rock garden then you would have two areas requiring drainage, each with a drain and pond liner.

What I have done is add some fill to improve drainage from the foundation, and added a drain in the area, as well as burying the gutter drains to a frech drain well away from the house. I want to cover the area between the structure and the pond with a gravel of a dark hue, which will set off the stepping stones we will put back in. The gravel will allow rain to flow to the drain- and I would prefer not to have plants and weeds growing up. The plastic was for weeds as well as well as drainage. I do not want to put in concrete. Due to shading the area does not grow grass well up next to the house- but weeds seems to do fine.

I am well aware of the pond liner options- having used it for the pond- but it is expensive and probably overkill.

I am not sure if the feedstore will know about use of the bentonite for this purpose.

As a builder of underground houses, I use 6 mil sheet plastic for the primary water shield, sometimes it's the only thing used. Works great. Our application includes rigid insulation between layers of 6 mil. We've never had any problems with foot or vehicle traffic with our 12" dirt overlay.