I don't get all the legal stuff, or potential legal stuff that might be preventing Smallville from using Superman's name. It's not like audiences are so dumb that they would confuse Smallville Superman with a silver screen Superman. Why couldn't we have both?
I think the idea is that why would you pay $10 to see Superman, in all his glory, on the big screen when you can see him for free on SMALLVILLE?
And how big a deal would a new SUPERMAN movie seem if he's on tv every Friday night anyway?
I'm not saying it's a good theory, but I believe that's the conventional wisdom.
Plus, of course, legal stuff doesn't have to make sense. It just has to be legal. If the CW doesn't have the rights to use the suit or the name or whatever, they don't have the rights, period.
You've pretty much covered the reason why the name isn't used, the suits at WB aren't about to let a TV show use a name that will be used in a theatrical release and earn them some big bucks. The core problem is, Clark isn't a schoolboy anymore, he works in the Daily Planet, and as far as I know, didn't he start work there so he could do superheroy things? This is why the show is silly, he can't fly, or wear the suit, at least make him fly, damn it.