I believe the standard wide 91 handguard will work. It will slide over the trunnion, I just don't have a wide 91 on me right now so I checked it with a G3K length one. The slim forearm won't work with it.
Yes, you can use a standard locking piece. On the bolt head, I believe the recesses in the trunnion will accept both standard rollers and half moon but I'm not positive. Maybe JFK will chime in. What kind of PSG1 trunnion are you talking about? If it's an RCM , then it should be fine .....75 Bronco can confirm that too (RCM owner).