685 on a GMAT Club mock with mocks averaging in the 660s is a solid baseline — you're closer to 700+ than you might think. Based on what you've described, this isn't really a concept gap problem. It's a stamina-plus-pacing problem, and those are very fixable.
Your CR dropping late in the session is the key signal. On the GMAT Focus Edition, Verbal comes last, and Critical Reasoning is cognitively expensive — it's exactly where fatigue shows up first. What you're experiencing is normal, but here's how to address it specifically:
The single highest-impact thing you can do is build stamina through full-length practice. Not just practicing hard questions, but doing full 2+ hour sessions where you're deliberately practicing mental endurance. Try taking 2-3 full GMAT Club mocks back-to-back weeks with no shortcuts, and log which questions you got wrong in the final 30 minutes versus the first 30. That data usually reveals whether it's a pacing issue (running out of time = rushing = careless errors) or a pure fatigue issue (slowing down = losing focus).
For the hard question gap: if Quant mistakes are concentrated on 705+ difficulty questions, I'd focus on drilling those specific question types (not all of Quant). Pick 2-3 weak topic tags — yours might be Number Properties or Combinatorics based on common patterns — and do 15-20 focused questions in those areas.
On GMAT Club mocks vs. the real exam: they're generally harder on Quant and the scoring algorithm differs slightly from the real test. A 685 on GC mocks typically translates to something in the 680-710 range on the actual GMAT Focus, depending on the section mix. Don't over-index on the score; use the diagnostic data instead.
You're asking the right questions — keep that analytical mindset on your actual exam too.