Lots of confusion in this post. You need to do some research on what septic tanks are and how they work. I'll leave it to others to say that your pumper is a hack and a moron (and taking advantage of you).
First, you don't want to "dissolve" the top layer. BTW, the top layer is the "scum" layer and the bottom layer is the "sludge" layer. The stuff on the top will be, to some extent, broken down by bacteria and settle down, in reduced form, to the bottom of the tank over time. The water comes in mixed with stuff, some of which settles promptly to the bottom and some of which floats until it is broken down and settles to the bottom or is pumped out. In the middle is water. That water will go under the baffle to the other side of the tank and follow a similar process again until the water floats out into the leaching fields (or the like). You want the water to be as free of dissolved solids as possible before it goes into your leaching fields, leading to a long life for them. The stuff that stays behind -- the solids -- you pump.
The danger with products that purport to "dissolve" the scum layer is that they can cause more solids to dissolve into the water that's flowing to the leaching fields, which is exactly the opposite of what you want, and then clog those fields. The point is to have the water flowing out to have as little dissolved solids as possible. The stuff in the scum layer includes oil and other stuff that will affirmatively-clog your drainfield. It's the last stuff that you want to be dissolving.
That's why you pump; to remove the stuff that is on the bottom and on the top of the water (sludge and scum). You are pumping to remove the solids; removal of the liquids is incidental; they have another way out. How often to pump is a function of tank size, wastewater volume going into the tank, wasterwater composition (lots of solids or not a lot of solids). Technically you need to pump when the scum layer and/or sludge layer reach a certain percentage of the available volume in the tank; everything else is a rule of thumb.
Second, a good design has a tee at the entrance and the exit. At the entrance to avoid stirring up the water in the tank; the whole point is to have the water minimally-agitated so that sedimentation can occur. It's like a container of orange juice. If you let it sit long enough, it separates -- sedimentation. If you shake it up again, it's all mixed up. More precisely, certain solids will automatically settle to the bottom and others will settle if they are retained long enough in the tank (how long they are retained is based on how much water is flowing through the tank). The more the system removes these solids, the longer your drainfield will last.
At the exit, you have a tee in order to have one last protection against stuff floating on the top going into your outflow pipe and thus out to clog the fields.
Third, the point of pumping a septic tank is to remove the very stuff that he is leaving behind. You pump out the sludge and scum, leaving a clean tank and starting the process over again.
Fourth, I'm not convinced that the issues you are having in your bathroom have anything to do with your tank. But I will leave that to others.