My take on your situation is that as you progress into a very long brevet (or do several brevets back-to-back) your core muscles are progressively getting weaker -- which is normal. In other words, you start to rely more and more on your arms to support your upper body weight, with your hands taking the majority of the blow (i.e., pressure) as a main point of contact. The moderate-aggressive geometry of your bike is fine for moderate long brevets but for ultra endurance (anything over a double century or 400 Km) you need to consider a more upright, comfortable position on the bike. Is there room on your fork's steerer tube to raise your handlebar to saddle level? You can also try to invert your stem or buy a new one. Four months for your hands to regain their strength/dexterity is unhealthy and overtime it will have undesirable consequences if you would like to continue randonneuring.
By the way, we saw this same hands condition last year with one of the guys in our team who completed PBP on a bike with fairly aggressive geometry. He could barely open his hands in the end which was quite disconcerting. It took him three months of visits to the doctor and physiotherapy to recover.
Last edited by Chris Pringle; 08-28-16 at 05:20 PM.