A single liner is the best way. We made ours with one liner.
Either would work. Mine is a continuous piece of liner that goes over the bog wall and into the bog.
If you do the small pond in the bog, I would have a whole bunch of holes coming out of it near the bottom, let as much water as possible flow out into the bog gravel down low. Some over flow is no big issue.
The main thing you want is for it to have the water go up through the plants and gravel, as much as possible. I had a top flow "bog" one time, ie water flowing over the gravel not up though. The surface of the gravel constantly clogged up with muck. I had a low drain coming out, most of the time no water came out it the water just flowed over the edge of the container.
The porous wall bog would have water flowing out into the pond but also some of the pond water would wander back though the wall. Unless you had a large flow of water out though the wall. When I was doing my "study" of bogs, tons of internet reading, the porous wall bog could also be built without water being fed into it, the pond water would wander though the wall and back out.
But imho, pushing some water flow through the gavel is the best way to get the bog doing a good filter job.
I just know when I built my pond with the bog as the only filter, a lot of people said it would never work. Glad I didn't listen, lol, my pond is a almost zero maintenance pond. The maintenance I do is yanking plants from the bog, they grow too good, grooming lilies. OW I can ignore it most of the time.