This is the second thread where I've seen it said that you cannot strengthen the pinky and that's rubbish. I started really focusing on building up my pinky a couple years a go and now I can do full bends with it.
Anyway... Marcus, just take it slow and easy. It'll come. I'm not sure how you feel about this but taking a very small dose of a mild muscle relaxer might help a bit but DO NOT take them all the time. Just enough to keep things limber to get through this rough patch. Alternatively (and much safer) you can get some ibuprofin or other non narcotic anti inflammatory medication. Obviously talk to your doctor first.
Other stuff, and I think I mentioned this last time we talked, is keep some of those ice/heat gel packs around and before/after practicing apply a cold pack for 10 minutes then a heat pack for 10-15 minutes. Taking a hot bath or shower will help loosen things up as well.
Then there is massaging the tendons and muscles. I personally do this with my hands but you could get one of those hand held massager wand thingies (some of them even provide heat and infra red therapy) and keep it near the piano when practicing. When you start cramping up give your arms and wrists a little massage until the cramp eases up.
When you aren't practicing you can carry around one of those foam stress balls to keep thing limber or a set of those Chinese silver zen ball things that you twirl around in your hands.
You also need to get some proper hand stretches from a physiotherapist. I got a bunch from my old PT and they work wonders but they are very hard to describe in text form and if done improperly can do more harm than good so definitely ask your doc.
There are also herbal remedies for keeping inflammation down and building cartilage that you can get at the health food store.
Drink LOTS of water and avoid caffeine/alcohol.
I don't have CP but I do have a condition that causes many of the symptoms you describe. It is a constant battle trying to keep my hands moving properly (as well as the rest of my body). I have to warm up every time I pick up the guitar and constantly stretch/massage my arms during a session otherwise I'll wake up in the middle of the night with my arms burning unbearably. I feel for ya, man. It sucks having all sorts of music stuck in your head but having your body disobey when trying to get it out. With patience, persistence and trying out different solutions though these types of things can be managed. Unfortunately though sometimes we just have to deal with a bit of pain to get to where we want.
To all you folks out there with limber hands who don't have to worry about these types of things be thankful and don't take it for granted. Take care of your hands and your body in general because once you end up injured or illnesses of the extremities strikes it is not only physically painful it can be very mentally painful. My hands were turning into claws and I managed to bring them back to life with a lot of hard work but believe me... it sucks.
Cheers.