I recently had a similar problem with the Cadillac. Random fuel starvation. Everybody screamed vapor lock but it was happening on cool days as well as hot summer.
They also told me to get an electric pump. After a couple trips on a flatbed wherein the car would immediately start after being dumped off, I started looking for problems in the fuel line and tank.
I used a plumbing snake camera like you describe and found nothing amiss in the tank. Air could be blown freely through the line from the fuel pump to the tank. I put aside the idea that the tank or line was the problem and a newly rebuilt, and properly tested, mechanical pump was installed.
The problem occurred again. No fuel was getting to the carb and none was visible in the filter after the pump. I unscrewed the tank drain plug and verified that the tank indeed had plenty of fuel. I disconnected the the fuel line at the fuel pump and installed a rubber hose which I placed in a 5 gallon gasoline can on the front floorboard.
The car started immediately and I drove 27 miles home. I replaced the tank and the entire fuel line and have had no other conk-outs since then.
My point is that if there is a problem inside the tank, you might not find it with a scope. Hopefully you find a smoking gun. First, I would install a rubber hose and a gas can to the mechanical pump and go for a drive. See what happens.
You may then be able to determine whether the problem lies upstream or downstream from the fuel pump. Perhaps a piece of solder or something is in there partially clogging the works.
Your over-pressurized electric pump may be able to overcome the blockage.
Using a gas can and hose removes a lot of variables. Maybe your mechanical pump can't suck fuel through the electric one. Did you replace the filter on the electric pump?
Also, I would not put any faith in the "new" Carter fuel pump. They are all garbage. Unless you test the volume output as well as the pressure, you cannot assume that it is working properly. 5psi seems pretty high. My mechanical fuel pump operates at 2.5psi.
You're better off looking for an original pump and rebuilding it.
