Some good advice, I'll add my 2 bits. With individual models you also have several smaller pieces that can be culled when not within the line of sight, if you have one giant mesh, you have to wait until the whole thing is hidden before it can be dropped. Or more common the mesh disapears because the pivot point is obscured…
1. This is pretty dependent on what you are doing. Generally though, keeping objects seperate and instanced is easier for you and better for the engine. If everything is seperate and you have a single object instanced around in several places, it only goes into memory once. If you have a bunch of the same object welded…