I've somewhat lost track of the TVEC versions, but Kirk will help you decipher them if you drop him an email, he is always helpful and prompt, but prompt can be relative, and during a sale it might be a day or so... be patient.
To answer the rest of the questions - OPINION ONLY HERE...
Diamond is a pretty darned good all-around orchestral library. All of the major instruments are covered, and most include all the necessary articulations, with a handful of extra articulations thrown in. The sound quality is very good, and the user interface is brilliant - very easy to use.
The sound will not be for everyone, it is what I'd call aggressive, but not in the epic trailer sense, more in the orchestra for pop/rock sense. If you've listened to any of Kirk's compositions that makes sense, it's what he does, or at least one of the things that he does.
The UI shows you which key switches you have enabled, how they are assigned, etc. Very handy! And while others now include the ability to purge unneeded samples, but Diamond was the first library I saw that included that feature. If your machine is getting up there in years this might be one of the more valuable features!!!
The built in reverb is ok, and when I first picked up the library I used it all the time. But time marches on, and there are very few Kontakt libraries with which I use the built in reverb. Diamond is not one of them<G>!
Concert Strings 2 and Concert Brass 2 are a step up from the strings and brass in Diamond. The sound quality is different, and to my ears better. It is more flexible, I can tweak it to be epic, and I can tweak it to be softer. I still don't use the built in reverb, but I probably could.
One of the reasons that I purchased Diamond, and later CS2 and CB2, is the ability to scale the size of the sections. I don't remember the counts off the top of my head, but each string and brass instrument is available in sections sizes from large (not epic large<G>) to solo within a single Kontakt patch. Man that is so handy!!
Over time Kirk added TVEC 3 and 4 to the menu. These add new features, some of which are so natural that I really can't recall when they became available, I just use them. There are some I haven't really latched onto yet.
Then came Spotlight String - it is a very different sound, and it is a lot of fun. I still have a ways to go to get it under control, but I like it a lot. It follows the same conventions that the other libraries established with respect to the UI, although it is a little busier, and I preferred the simpler look... but again that is just one person's opinion.
I left out Pop/Rock Strings. It is very similar in tone to CS2. But in includes some articulations that are used primarily in pop music productions. To be honest, I wish I could get by with just CS2, but that's just me being lazy and cheap. Pop/Rock Strings is licensed for the free Kontakt Player.
I have, from time to time, suggested that Kirk treat the winds to the same Concert XXX approach that he used for strings and brass. While the winds in Diamond are certainly useful, they are not quite up to the standard he set with CS2 and CB2. Perhaps that is coming?
Now none of these libraries are going to replace Cinesamples or Spitfire or 8Dio or Audiobro or... and I think that's part of the point. None of these libraries can fairly be compared as apples vs apples. I really like the Audiobro sound, and some of their features have yet to be duplicated. By the same token, 8Dios Adagio and Adagiatto have, to my ears, the most convincing bow change legato, and the next library (forgot the name) has that soaring JW legato... gorgeous! Cinesamples and Spitfire have libraries that include the room and the rooms they include are just jaw droppingly cool. And both of them have a cool take on articulations, with neat features that separate them from the pack. The Universal Articulation Change thingy that Spitfire is promoting is especially intriguing.
So there you go, I can find reasons to own and use all of their libraries. There is no one size fits all. Heck, I didn't even get to Vienna or EastWest, both of which are pretty amazing as well, though neither is a Kontakt developer.
Which brings us to the question "should I purchase KHS libraries" - especially when he runs a sale.
My answer to that is "it depends".
I started with GPO, and I have no regrets about that purchase. But for me Diamond was a HUGE leap forward. I was able to create much nicer sounding mockups - although the amount of time and effort required increased too.
And I will not stop with my KHS libraries... all the libraries I listed above bring something else to the table.
So if you own Cinesample or Spitfire or EastWest or Vienna or 8Dio or Audiobro or Cinematic Strings or any of the other high end libraries then I am not sure why you'd buy KHS unless there was a specific feature you were looking for, in which case you probably aren't reading this thread<G>! And in that case you probably are going to buy it because it fills a specific requirement.
BUT, if you are upgrading from GPO or a similar "starter" library I think KHS makes a lot of sense. It is priced competitively (especially now), and it is very easy to learn, and it sounds lovely.
Did I bury the lead again?