Walls
Only above ground bogs, just like ponds, would need walls. The higher you go the stronger the wall needs to be. For example, 6" above ground can just be a course of concrete blocks, no footer of any kind and no poured concrete. To go 16" high you'd probably want to fill the two courses with concrete. Filling them with just clay dirt and packing could give you years and years of service with a little movement over time, could bulge out next week. The poured concrete is more like insurance. When you start thinking of 20-24" you really need to be thinking blocks filled with concrete, and some rebar wouldn't hurt. In the 24-30" range rebar become important. 30-36" rebar is really need and footers become important. above 36" and you really start to need good standard building practices for your specific area, including footers, proper rebar and proper building practices.
You can use any building practice you like, it really only about risk and how much movement you consider acceptable. Using a liner can allow for walls to bulge and heave without any real harm, as long as you don't mind the look. Depending on climate and soil conditions you could get years of service. Most ponds don't last very long so there's little reason to build something to last forever.
Concrete ponds that have a painted on liner are very different, They require an extremely stable structure. Even the smallest heave or bulge will rip the membrane and cause a leak that really can't be fixed, only patched until the structure moves again.
Drain
Sounds logical. Bogs do indeed fill with waste and eventually will be completely full. For most ponds that's not really a problem. It's actually one of the bog's best features. A filter that lasts for 10+ years and never has to be cleaned. Cool.
Drains would be kind of cool if they worked, at all. "But I see dirty water come out!" Insignificant I say.
Distribution pipes
Another "improvement". Sure sounds like a good idea and they do no great harm. The trouble is water doesn't care about where you want it to go, it goes where it's easiest. Even if that means coming out the pipe and running right back along the pipe in the opposite direction you thought, that's where it will go. If it can all travel down the pipe and pop up at the end right next to the outflow, bypassing 90% of your bog, that's what it will do.
There's a Catch-22 with these distribution pipes deals. The holes have to be pretty large to keep from clogging. And given the number of holes I see often added means that maybe 10% of the holes can handle 100% of pump's outflow, so most of the bog, will be bypassed because flowing down the pipe to wherever is closest to the outflow is always going to be the easiest path.
It is better imo for the water to take the longest path. Every distribution pipe deal is going to reduce that. Not to mention that water falling onto the surface of the bog is way better bio filtering in just that small area than you'd get in the entire rest of the bog. Pile up the rocks at that inflow and increase capacity 10x (or some big number).
Settlement chamber
A bog is primarily a settlement chamber and they do it very well. Yeah, they do a little bit of this and that too, but not very well. And bogs are all about clearer water, not better water quality. Waste accumulates and decays in the bog and the results of the decay ends up in the water column just as sure as muck piling up on the bottom of the pond.
My bottom line
Bogs are great at what they do. Love them. For most water garden owners they're by far the best possible filter because most will hardly ever clean a filter. So for them you can't beat a bog. But they're really bad at other stuff, or at least there are way better choices today.
IMO a bog is a hole in the ground 12-18" deep filled with small rock and has water pumped in one side (just a single pipe on top) and water overflows another side. That's it. It's that simple.
