Yes, it will.
Yes, it is compatible with any standard AR-15 buffer system as well as the JP Silent Capture Spring.
Yes, it will.
No, it will not.
Yes, the original HK 416 and MR556 buffer tube is a carbine length milspec tube. You will be able to use any milspec AR15 buffer tube such as this Aero Precision Enhanced Carbine Buffer Tube and use your original buffer and spring.
A carbine buffer is 7.5" in length and is generally speaking the shortest you can get unless you use a proprietary system. I would recommend the Aero Precision Enhanced Carbine Buffer Kit with H3 buffer, found by using the link below:
https://www.opticsplanet.com/aero-precision-enhanced-carbine-buffer-kit.html?_iv_included-accessories=h3-buffer
Yes, the Aero Precision Enhanced Carbine Buffer Tube is compatible with the Law Tactical Folder.
Mil-spec Dimensions
Standard mil-spec carbine buffer for AR-15 applicants.
Yes, it will fit any standard AR-15 or AR-10 lower receiver. Please be aware that if you have a billet lower receiver, you may run into fitment issues because of the "Carrier Support Extensions" that extend into the lower receiver.