Regarding the hard fork to make block withholding impossible:  It will never happen.  AFAIK, that hard fork would also break all existing ASICs.

Granted, a hard fork does NOT need 51% mining power to be implemented.  What matters with a hard fork is the market (people who accept/trade Bitcoin) moves to the new fork.  But unless I'm mistaken, the secret data used to prevent block withholding would be incompatible with all existing mining hardware, which will immediately make such a fork very hard to achieve.  It would more likely kill BTC entirely due to the competing standards.



As for variance/luck/withholding:  I'm monitoring closely users with bad share/block ratios.  Unfortunately, there's really not much else I can do.  It was around June 9th we had a pretty big dive.  But then we had a decent rise that roughly offset it before going into 3 days of consecutive very poor luck, caused mostly be a few 5 and 6 hour rounds.  However, the pool was at 96.8% luck for the last month before the dive, and rising.  99% is "neutral" in the sense of expectation, since you would expect a ~1% orphan rate which means 100% is actually slightly lucky.

The pool has not grown significantly, nor has the difficulty risen in the last 3 days.  The current dip appears to be luck, not withholding, based on the fact that the long rounds mean all the prior speed (11-12 PH/s), which was performing right within expectation for most the last month, weren't finding blocks for 5-6 hours along with the new speed that joined.