Interesting post
My response in a single word.
Overtraining.Its far too common with abs. The abs are a muscle, just like all other muscles. For example, would you apply the same routine you described to your biceps, back, legs etc.?
All muscle responds in the same way, i.e. fast twitch, slow twitch is the same in the abs as in the biceps and so on. The basics of muscle training applies to the abs as it does to all other muscles. When you train the biceps, you will roughly do 5 sets of heavy loads (~8-12 reps) which is the optimum combo to trigger new muscle growth - any more or less will fail to trigger new growth.
In the same way, once you've reached the trigger point - any further training will tend to strain your muscles and a process of protein breakdown begins - i.e. your muscle will eat itself to maintain function (this is overtraining).
In the same way, if your overtraiing the abs - your muscle will simply not grow. In other words, no matter how much further training you do, the muscle has given up growing. This means visually, you will not see any results, even though the strength may have increased.
In my opinion, its best to train the abs just like any other muscle (bodybuilding style) - this will give you the growth, hardness plus some serious core strength.
On other occasions, do high volume abs training, similar to what your doing atm, this should give the sharp and crisp definition of the abs to complement the muscle growth.
The most important factor throughout (which any trainer will tell you) is REST. So between training, make sure you give the abs time to rest, plus this is when the actually grow (during repair) and this is also the main time you should be eating pleanty of proetin and remember the carbs and fat - don't exclude fat from your diet!
Hope this helps, good luck.