1: yes, Grenades have parts, they determin the delivery mechanish, the fuse time, the radius and damage, but they don’t get “manufacturer parts” to compare…they simply have good or bad stats
2: All unique guns (Legendaries, uniques, seraphs, Pearls) have a fixed barrel, and almost all Legendaries have their manufacturer matched barrel (Exceptions being a few shotguns, the Ogre, Hammer Buster and maybe one or 2 others)
While all uniques have a fixed barrel, most don’t have a MATCHING barrel (like I said, except for legendaries)
All other parts are un-fixed: any unique can have any accessory, sight, stock, grip or exhaust (for RL)
Some guns have special rules: The striker will always spawn with one of the strongest sights available (never Torgue), the Buffalo and Elephant gun will never spawn with a scope, the Rapier will always spawn with a blade, but it will not override a possible accessory (Same with the Law). Maliwan guns will ALWAYS have an element, All Torgue guns will be Explosive, Jakobs guns are Never elemental (except for a few oddballs) Some have locked elements. All Moxxi guns will always be elemental, even the non-Maliwan ones. The Infinity cannot spawn with Slag.
…I can’t think of other things right now. If you have specific questions though, don’t hesitate 
Forgot about him. I think he’s the only one though.