This is good news, he sounds like the right man for the job.

I had a feeling the right one wouldn't charge for the removal of the maindrain.

A couple of thoughts. The walls might be a masonite product rather then actual wood. But the concept is the same.

I suspect the construction of the pool is probably what we used to refer to as the "popular mechanics" design. Lots of these wood pools were built for many years and are fairly common. It's sort of a home owner's design that can be constructed out of materials purchase at a lumber yard, and with only a few pool specific items added. Coping/extrusion and the liner being the two biggest ones.

The biggest factor when replacing a liner with these pools is actually how it was orginally constructed. Many of these "wood wall" pools were constructed in a way that makes removing the water from them precarious sometimes. Essentially when the pool is first built it is filled with water and backfilled behind the walls at the same time. This is how the pool is able to be built out of wood walls, with less then adequate support . As the pressure of the water filling the liner reaches the areas of the walls, the back fill (dirt) is also added, thus equalizing the pressure. If the reverse were done, and the pool was backfilled with out the water in it, it would be too much pressure. The same goes with the water if the back fill isn't added.

Afterward the walls become much more stiffened with the addition of a deck. Especially if it's tied into some type of coping which is attached o the walls. It's the first liner replacement or the first time the pool looses a significant amount of water (off the shallow end) that how much strength the deck has given the walls, is known.

Let me ask you this:

Did he ask you to not drain the water out of the pool and to actually keep an eye not it, as he awaits the delivery of your liner?

Did he mention that there's a chance that re-filling the pool maybe needed to be done via trucked in water?

More importantly, how much water is in the pool now, and when is the estimated time of the replacement?

I ask because sometimes when water slips well below the middle height of a wood wall pool, "bending" or "bowing can happen. The middle of the longest stretches of wall are obviously the most vulnerable. Once this happens it's imperative to continue with the replacement as expediently as possible. Empty wood wall pools are not good to have in that condition for long periods. Some hold up fine, some "bow" a little each time water gets below the walls, whether through a leak or liner replacement. It's best to keep water in the pool right up to the day the liner is being replaced. The shortest amount of time the pool is void of water, is what an installer should be shooting for. Unless he know the pools ability to hold it's own with out water in it.

Since your pool has had a liner replacement at some point we know it has withstood being empty at least once. Which is a darn good sign.

As far as a 20/20 vs. a 28/20 he is correct, 20 mill walled liners do conform more easily to variations. Are the corners of your pool (the actual dimensional corners or "shape"), wide sweeping arcs or closer to 90 degree corners with perhaps a small "clip" or short 45 degree angled section? Because this is where a 20 mill walled liner is most effective in getting a better fit, via it's stretching. If the actual corners of your pool aren't arcing corners, and more abrupt it probably is the better choice.

The only draw back to a 20 mil wall is the fact that (not commonly known), a liner most often fails above the water line. Due to UV damage and chemicals gathering on the surface. Between these two factors and the liner not staying as elastic from not being underwater are the biggest reason for failure. That was the reason for the thicker wall but yet not a thicker "floor". It's not a huge factor, and liners for years were all 20 (20/20). Hence the equivalent warranty. I think it was more of "upgrade" type marketing move by the liner manufacturers. Would I recommend one , sure , thicker is always better for the walls, but if it would have hampered a proper and correct installation, I would never opt for one, nor recommend one.

Any whoo........ just a few more thing to consider now that we know more about what's going on. Nothing bad, just more info imparted to you via this forum.

Good Luck.