I think there is a big misconception when it comes to CRHing. The misconception being that there are boxes and boxes of coins containing wagon-loads of silver that have been collecting dust in a vault at the Fed or coin couriers' facilities for years. "If I could just get past all these newer, mostly skunk boxes, I could get to the 'good' stuff", is a very common thought among CRHers. I personally believe - but have no way to prove - that this is just not true for the most part. Sure, the law of averages says there are a few boxes out there somewhere that haven't seen the light of day for 10-20+ years, but for the most part I think those boxes are long gone.
What you are really searching for when you CRH, is recently dumped silver...and hopefully a big collection. If no silver gets dumped into your system, then you aren't going to find any. Its as simple as that. If you have a big-time CRHer(s) and/or many small-time CRHers dumping into your system (and I'd guess that every system has multiple CRHers dumping into them regularly), then you have even a smaller chance of finding that elusive silver as their dumps "pollute" your supply.
16 skunk boxes in a row is a lot, but more common than you may think. You might get to 100 boxes or more without a single silver half dollar. Your next box may have 100 keepers. As I said above, what your waiting on is that big collection to get dumped and at least a portion of it find its way into your boxes. There is no way to predict where and when that collection is going to get dumped. As for the marked coins, just ignore them. They're very common. For the most part us CRHers just trade each others dumps back and forth, hoping some silver gets mixed in somewhere. I would venture to guess that if it was a rule that you had to mark each clad half everytime you searched and dumped them, 90%+ of the coins would be so marked up that there would be no room on the coin to add your own mark. Again thats just more speculation on my part.
Only you can decide when "enough is enough" and give up. I do know that you cannot win if you do not play. But, there does come a point when it just wouldn't be worth the effort, and no one can decide just exactly where that point is but you. For example, I like to fish. I've heard it said many times that, "A bad day on the water is better than a good day at work", or something along those lines. With that said, if I had fished the same lake many times without catching a single fish, I'd have to at least consider switching lakes. If switching lakes didn't produce different results, eventually I'd look for a new hobby.