Ok to answer your points in order-

Yes the OE silencer is reflective, and contains the catalytic convertor before the baffling. By doing this, the amount of baffling needed can be reduced, but it isn't a very efficient system for performance.

Which brings us on to the next point; I believe that the majority of aftermarket silencers for the Tenere are designed as twin units to fit in aesthetically with the bike once the neat OE unit is removed. As far as weight, and the removal of it goes, you aren't helping yourself by fitting twin silencers, when one would suffice. However; fitting a single silencer to the Tenere throws up both aesthetic and flow problems of it's own.

If you want the most efficient flow you have the silencer on the left of the bike, but this leaves the right hand side of the bike looking very exposed and unfinished. As the bike leans to the left on the sidestand you get to see the dirty side of the silencer whilst the bike is at rest.
If you decide to fit a single silencer on the right hand side of the bike, then you have to add a couple of bends into the system at the point where the gas speed it at it's slowest, so you risk adding more back pressure to the system. This is also true of systems like the Leo Vince link pipe where there is a smaller diameter secondary pipe going to the RHS silencer. The way this secondary pipe is sprued off from the primary will have an effect on the turbulence of the gas flow at that point, again affecting back pressure. I can only speak of the LV system as I haven't had a look around the others available.
I wasn't too enamoured with the LV system and thought it heavy and certainly the RHS silencer is only there for aesthetic balance.

The OE silencer isn't a dual silencer, but a single silencer with twin outlets. By designing it as such the manufacturer can make use of the space efficiently and stylistically it would tick boxes with their design department.