Try PM-ing Gabriel at Gemini. Regarding the "NebulaCcompressor" thing, all I can say is follow the instructions to the letter. You are making a new dll and xml that is for the NC presets in the library. I use Nebula Setups to have that dll load ONLY the NC presets, but you can also use the dll to load the entire library to keep it less confusing. The side-chain presets are set up to use external routing, it doesn't have the side-chain filter on the preset. But you can use any simple EQ (like reaEQ in Reaper set as a high-pass) and do use the typical Reaper side-chain routing. But about the subtle differences between all of the other presets..... You'll need Gabriel, I actually only installed a very small part of the library, there were just too many options for my simple brain to deal with!
There is a detailed instruction for creating the Nebula Compressor copy of Nebula in the manual on page 4. I need to know, which step is unclear.
G2500 Manual Page 4
On page 3, there is a detailed description for the categories. RE4 and COM are not different section, but RE4 is a subcategory of the COM category. As you can see in the manual, the COM, COP, COB, CBP etc. categories contain different compression styles. All presets are contained in those main categories as copies with different settings for different compression styles, e.g. FeedBack compression.
G2500 Manual page 3
Most of the programs are for flexibility, so that every user has the best presets for their system. For example, if you only work in 44.1kHz, you won't install the other sample rates, which already gives you much less choices in the preset browser. For most users i would recommend using the "Basic" installation. Then i would recommend you load "NC" program in your target sample rate, and see if your system can handle that, and if you can load enough instances for your needs. If not, you might wan't to try "ME" presets, and if they still are taxing your computer too much, you could try the "NL" presets. If those still are taxing your system too much, use the "RE" programs, e.g. "RE4" which use the least amount of CPU.
In the following section of the manual, it is described, how to use the sidechain. It also depends on your DAW. If you tell us which DAW you are using, we could tell you how it works for your DAW specifically. Basically the sidechain function gives you the ability to determine, what the compressor will react to. This could be a copy of the signal with certain processing like filters etc. or a certain instrument like a kick drum. If you process a copy of the same signal which you want to compress using the an SCF program from the G2500 library and send it into the sidechain input of nebula, you'll get frequency dependent compression like the hardware does using the THRUST mode.