Slow process of diagnosing this: information here is a summary of my treatment and helped by top neurosurgeons, physiotherapists, couches and other professionals. You can read my diary below. We are trying to get me back to games fast.
[24th June] general doctor order me 2 weeks of rest with anti-inflamatory painkillers
I am doubtful about this because I cannot see inflammation. I try it and fingers developing some pain after being in the mold 1-2 days.
[26th June] no inflamation whatsoever so my couch instructs me to prepare me to use the hand normally
Stiffness in the little finger and ringer finger and pain in the little finger when trying to put into fist after being a long time in alu mold. Too stiff shoulders: cold in shoulders relaxes the ring finger. Hot in hand relaxes the stiffness in little finger and ring finger.
I am feeling totally normal again and sleeping with alumium fold again so the fingers get oxygen during sleeping. Goal is to get better in one week like pros.
28th June
Bandit-style device made my fingers feel stiff. Cold makes my little and ring finger feel stiff. So I got compression socks to hands which increase the blood flow and makes the forearm feel more warm, feels good.
29th June
Having the aluminium mold on during the night can make the extensor digitorum in the little finger darker after night. After relaxing it, the darkness go away and this stiffness get lessened but the spring-feeling stays.
30th June: little finger starting to feel normal after adding exteremely strong compression to forearm
I realised that I can get away with the jumpiness/springiness/stiffiness/powerlessness of the little-finger by pressing tight at certain points of the hand. So I tested a rubber band: the blood-flow gets totally intercepted but the little finger started to work like normal for a while! Wohoo! The colour of my hand changed and oddly my middle finger started to feel odd, I don't know how this can work -- somehow extremely tight compression in the forearm released some tension or something like that and you need to be careful not damaging other fingers.
2th July: BandIt arrived
Rubber band method and BandIt are about the same method but BandIt does not block your blood flow that much, fingers staying white i.e. not getting red/black with BandIt. The coordination of my little finger has returned back due to playing with rubber band and I believe BandIt is the safer method equivalent to rubber band that can be used during any sport.
Misc information
Different Stretching Methods
1st method
Push-ups with fingers feel very very good to the fingers, this is because deadlifts with about 90kg require a proper warmup.
2rd method
Rise-bowl-scrutching-method suggested here warms up the whole hand and actually simulates the handles of different sized rackets -- unfortunately the shape of crutching rice and crutching handles of rackets are not the same so not specific to the ring-finger-little-finger-area.
3th method: Static stretching
Stretching like here but at least 30 seconds' interval so it takes at least 20 minutes to stretch just one hand. Excellent stretching also here by a rock climber, it feels excellent!
Different relaxation methods
I. method: resting during rock climbing
A rock climber shows here advanced resting techniques for hands. It may be possible to adjust to other sports: hands slowly down and then hand up here crutching hand push the fresh blood back to the hand.
Different coordination methods
A. method
Powerball is not sufficiently powerful/large to train my fingers on my serving hand. It certainly makes fingers a bit more flexible in the left but I think the main thing it helps with is coordination, it actives areas of hands which may not be under normal usage during day and sporadic spinning requires proper coordination or bad sounding noise generated.
Related Threads and material
Rehabilitation for Ulnar nerve, awesome stretching movements in the video "Ulnar Nerve Flossing Exercise - Amazing Results - Kinetic Health" here.