Cheat code? Not really. Fewer flight training hours required? Yes.
Any restrictions? Not sure what that’s asking. Private glider removes the sport limitations of altitude, night, etc, but private pilots still have limitations - not flying for pay, IMC without IFR, class A without ATC clearance, etc.
Not unsafe at all. Same proficiency standards must be be.
A DPE/SPIE will tell you that a sport proficiency check has the same standards as a check ride. I’ve had a SPIE tell me that a proficiency check is actually more aspects than a check ride. So the scrutiny level is absolutely there.
Yes, you can train for private glider and do the check ride for it. The issuance of your glider ticket would have your underlying sport airplane as an endorsement. You absolutely do not just automatically get a new rating, such as ASEL-S. You’d need to receive training in an airplane per 61.309 and 61.311 first. While no minimum training hours are specified, you must receive and log training and demonstrate proficiency on each of the requirements specified in order to be recommended to a second CFI for a proficiency check - which is nearly identical to a check ride. For the ASEL-S addition to a PP-G with your sport endorsement, an 8710-11 is completed by the two CFI’s. You’d then have a PP-G, with two underlying sport endorsements. Nothing is “surrendered,” in reality; just built upon. Hope that helps.