This buttstock is meant to install to an adjustable buffer tube and per FAB Defense, it uses a "Unique patented “Anti-Rattle” mechanism, fits perfectly on MIL-SPEC and commercial after-market buffer tubes".
Yes, the FAB Defense AR-15/M4 Stock QD attachment point is compatible with the Magpul QD attachments.
Yes, you can install the Adjustable Cheek Riser with Picatinny Rail for GLR16, or you can buy the whole package together.
https://www.opticsplanet.com/fab-defense-adjustable-cheek-riser-with-picatinny-rail-for-glr16-stock.html
https://www.opticsplanet.com/fab-defense-ar-15-m4-stock-with-adjustable-cheek-riser-battery-storage-and-rubber.html
If it is installed to a buffer tube that has length of pull adjustments, then yes, the stock can be adjusted forward or backward toward the firearm.
This stock will mount to any mil-spec or commercial buffer tube. You will want to confirm exactly what style of buffer tube your Carbon-15 has and if it is a mil-spec or commercial buffer tube, then it will fit.
Yes, it will fit your commercial buffer tube as well any mil-spec buffer tube.
FAB Defense lists that it will fit both.
No, it is just the buttstock.
If your Diamondback DB10 has an adjustable buffer tube it will likely fit as the two most common patterns are mil-spec and commercial and this stock will fit both of those. If you have a fixed buffer tube with no adjustments, then this stock will NOT fit without changing the buffer tube.
This stock is meant to fit a mil-spec or commercial buffer tube. You will need to reach out to Mossberg directly to verify what style of buffer tube they use on their 715.