Hi mawile,
Well there’s some good news in your feedback anyway 
Unfortunately this is fairly “standard” practice. It’s not unusual at all for some of the replies to questions that the members here have learned to ask are along the lines of “nobody has ever asked that before”. I get this all the time. My reply of course is that “I’m asking now” and "this kind of information is supplied by other manufacturers so I wanted to find out how much of the quality specs of your mattresses you are willing to disclose so that I can make better comparisons.
Things are gradually changing in some corners of the industry but only when these types of questions become the norm and manufacturers feel that lack of disclosure will cost them in terms of profits will things change a little faster. At this point marketing is still overwhelming more meaningful information in most of the industry and in some areas of the industry they are also changing for the worse. The key is for consumers to ask these questions more and more and insist on answers as part of every mattress purchase. If manufacturers have no “need” or “incentive” to disclose information about the quality and durability of the materials in their mattresses, and as long as consumers are willing to believe marketing copy regardless of the truth behind it and “buy” mattresses based on these marketing stories … then things are much less likely to change at all. This is where many (not all) of the smaller manufacturers that sell directly to the public (or in some cases wholesale) can shine and they are actually eager to provide this kind of information and grateful to consumers who have learned enough to ask.
While I like what Diamond is doing in many cases … as you can see in post #2 here I have tried to find information about some of the missing “specs” in their mattresses only to discover that they are not responsive at all to questions from consumers and in the end I gave up finding any more detailed information about them. I think this is unfortunate and a major marketing and credibility mistake on their part. Hopefully some of their retailers will make this information available.
I’m glad to see you found the soft version though!
Waterfall is just a type of cover, typically in a Eurotop, where there is no seam in the top edge and the fabric “flows over” the sides like a waterfall like this or like this instead of having a tape edge or a seam on the top.
A tight top like this will have a tighter construction that has a tape edge which will compress the materials and make them a little firmer.
Also, since my partner weighs 253 lbs and I weigh about 125 lbs, would a soft mattress be a bad idea for him?
That depends entirely on the total mattress construction and which parts of the mattress are soft and which are firm… A soft support core would not be a great idea (it would probably allow his heavier pelvis to sink in too far) and a comfort layer that was also too thick and too soft would also not be a good idea both for durability reasons and also because of the risk of alignment issues with a heavier person. A little bit of softness would generally be fine in the comfort layers. With heavier people it is also very important to make sure that the materials in the top part of the mattress especially are durable or they will soften and degrade faster than they would for a lighter person no matter what materials are used. You can read a little more about ways to accommodate two people with very different needs or a larger weight differential in the first part of post #2 here but I would use your testing in combination with the knowledge and experience of the manufacturer or retailer to help you determine whether a mattress would work for both of you in both the short and long term rather than “theory”.
Phoenix