The OP's suggestion is exactly the same as saying TN 12/18, just differently.

Meaning if you go with TN 18 and say Prime gives a +6, you may as well say TN is 12.

As for CL, its not hard. You choose a CL based on HD/lvl of the opponent or based on how difficult the CK thinks it should be. A rule of thumb guideline is in paragraph 3 on page 110.

So you decide on the CL and add it to TN 18, or 12 if its a Prime attribute, or you can just add a +6 in to the modifiers for the check/save, whichever, they both work.

Even in 3E a DC 15 is not simply a DC 15 check, modifiers probably come into play, which changes the final DC. You also have DC 10 plus spell level, and other base DC's you can use in 3E, just most often you start at 10 or 15. Hmmmm... 10 and 15 equals 12 and 18?

Why 12 and 18? Because it is the chosen range for the mathematically significant difference on a d20. Its actually 5.somethingsomething, but it rounds off to 6. They then chose 12 and 18 as the base line. You can use 10 and 16 if you wish, as long as you want a mathematical significance and use the number 6. Others use 5, but it does not meet the mathematical definition of "significant difference", however 5 is only off of the definition by something like .6, so does 5 really make a difference from 6? Apparently people do, since they want to change it to 5 from 6. Some obviously do feel 6 is truly significant, since they feel its too much and 5 is good enough.

So the CK decides to use either the HD/level of the opponent (same as using spell level in 3E) or determines if its in the easy range (1 to 5), difficult (6 to 10), or worse, and adds it to the TN of 18 (DC 15?) for the final TN/DC.

So not difficult, once you understand it anyways.