You're right, for the initiate, beginning to understand latex is very confusing.
The key things to remember are:
1) There are basically two ways that latex is MANUFACTURED.
a) The Dunlop method. This is the original method of producing latex, and is generally manufactured this way in places like Sri Lanka (sometimes called Jungle Latex), although there is at least one manufacturer in the US who produces Dunlop latex. Dunlop latex tends to be firmer and more dense than Talalay latex.
b) The Talalay method. This is a later method of manufacture. It is more expensive, to make, but produces a more consistent product. Talalay latex is less dense, and "bouncier" than Dunlop latex.
I suggest you do some google searches of "Dunlop latex" and "Talalay latex" and you will find some videos and web pages explaining (and showing) the manuracturing process for each type. What you will see is that some websites toute "Dunlop" as being more natural and better, and others say that Talalay is the finest. Don't pay attention to any of these claims. They are both fine products and one isn't "better" than the other, though they do have some different qualities.
2) There are two TYPES of latex: (someone jump in and tell me if anything I'm saying is wrong)
a) Natural latex. This is latex that is 100% comprised of rubber from the rubber tree. Most if not all Dunlop latex is 100% natural (someone jump in and tell me if I'm wrong)
b) Synthetic latex. This latex is 100% synthetically produced and does not come from the rubber tree. Some of the big "S" brand companies produce latex that is 100% synthetic.
c) In producing latex, some manufacturers of Talalay latex will BLEND Natural and Synthetic latex. The reasoning is that blended latex is more durable.
SO: There is Dunlop latex, which is 100% natural, from the rubber tree, and there is Talalay latex, which can be either 100% natural or a blend of natural and synthetic latex. I'm not sure whether the 100% synthetic latex produced by the big S companies is considered Talalay, I don't think so. I think they use an entirely different process.
3) Latex comes in different FIRMNESSES. These are usually described in therms of "Indention Load Deflection" (I think that's the term) or ILD. The lower the ILD number, the softer the latex is. The higher the ILD number is, the firmer it is.
For your father's topper, the reason they said they glue two pieces together is that latex molds are not made in the shape of a king sized bed. They have to take two latex pieces and glue them along an edge to make a king sized topper. This in no way affects the feel of the latex or its quality. I wouldn't be worried about it.
One way to know that you're getting what you ordered is to order your latex bed, or topper, from a company that sources their latex from a quality manufacturer. When you're buying, for example, from Overstock.com, you have no idea where they got their latex and in fact, you can't be assured that you'll get the softness or firmness you want.
Most of us who are buying latex beds buy from companies that source their latex from Latex International:
http://www.latexinternational.com/index.php
Which makes very high quality Talalay latex in both 100% natural (all from rubber tree) and blended (part natural, part synthetic).
I suggest you look at and read up on these two websites. There is a lot of information on each website, and both source their latex from Latex International.
www.flobeds.com
www.sleepez.com
There are other companies that make Dunlop latex beds, but I don't know as much about them. Some people who have really bad backs, or want very firm mattresses, prefer Dunlop latex mattresses. I would hazard a guess that most of the people on this forum end up buying Talalay latex mattresses. I, for example, have a terrible back, and hip problems. Before I got my latex bed I suffered from excruciating pain sleeping at night, plus numbness and tingling in my hips. Now I have a blended Talalay latex bed, with latex sourced from Latex International, that I purchased from Flobeds. After buying my Flobeds mattress, I bought a blended latex mattress for my daughter from Sleepez. Both great quality mattresses, and I have no more numbness or tingling in my hips. Back issues are not 100% gone, but greatly alleviated by my latex bed. Sometimes my time sleeping at night is the least pain I feel in a 24 hour period.
I hope this is enough to get you started.