Will Main Hand only weaponry be 'Mog"ifiable to the offhand slot?
Mainhand can only be applied to mainhand, and offhand can only be applied to offhand. This is partially due to the restrictions on certain types of weapon art.
Again, these are our first tentative steps with Transmogrification, and there's room for modifying the rules later on after the system is introduced, and we see how things shake out. Heck, the rules aren't even quite set in stone right now, and might change further before the system is introduced.
Btw, if WoW was designed to not be "silly" why have the items in the game in the first place o.o?
World of Warcraft has a design aesthetic that limits silliness, not omits it utterly. We all like a laugh now and then. That's why we have so many fun novelty items, easter eggs and humorous quests. Still, with a handful of exceptions, there's been a pretty firm line drawn between what amounts to a novelty item (which we anticipate players showing off occasionally for a laugh, or to enjoy on a temporary basis), and items that we intend players to use in their day to day activities.
It's that handful of exceptions that are the sticking point. It's the items like frying pans, cleavers and rolling pins that we're not sure we want players running around with on a constant basis. The jury is still out on that.
I'm sure that there will be those that still find ways to make the most ridiculous outfits possible despite any precautions we take. We can't possibly anticipate every permutation of gear combinations that people will come up with, but we can strive to design the system so that there are fewer completely ridiculous looking characters running around. Yes, that's partially subjective. Yes, we also have a design aesthetic that, even with the introduction of the Transmogrifier, we still want to adhere to. No, I don't care if you personally think that any given tier set looks "silly"; that's not what's at issue.