I am not fantastic at combos and finger dexterity myself and tend to prefer slower paced fighters that focus more on spacing and punishing with few hits but I do like most fighters to an extent and my execution has increased over the years.
Simple answer for dexterity is practice you really will find no easy fix sorry but its gonna take a long time. I dont know how familiar you are with a fight stick but I know when I first started it drastically reduced my abilities and even now while I can interchange to a reasonable extent I am more comfortable with a controller in most fighters. Really though you want to become familiar with what you are using so just play with it and dont worry to much about combos you cannot do even if they are very easy ones.
For doing combos once you are adapted to the pad or stick I tend to use chunking as in take a small part of the combo and practice it especially bits you have trouble with even if its say cancelling a hit into super. Then eventually put it all together. What will happen is you will be able to do parts and then fail again when you try and put it together (at least I do) but keep at it and you will get better. Sometimes a break helps as well.I have had instances where I could not do a combo to save my life so gave it a rest then the next day I did it straight off the bat no sweat and unfortunately the reverse is true but eventually you acclimatise and it stops becoming a challenge and more a tool. Also once you can do it satisfactorily one side do it the other until you are happy there.
As for the carpal tunnel I dont know anything about you so its hard to say but I know I got it when I was to tense especially when trying to mash out combos because I was not comfortable with the timing (usually you dont need to be as fast as you think and if you do there are easier ways to go about it due to the way inputs work). So I would just say get accustomed to using the stick if that is what you want to use, find a comfortable way to hold it (I often change hand movements when using it) and just relax.
Just checked challenge 7 (never played extend) it doesnt look hugely complex but definitely not a noob combo seriously I have seen noobs play (and these are people who have been gaming for most of their lives) many cant even finish the tutorial in MKX or GG Revelator. Then again I remember being a true noob when I couldnt even do a fireball although at that time the way to do it was not common knowledge and I had never played anything that required movements like it and no notations didnt help as it did not show a quarter circle only and arrow to each point.
Oh and you are playing the wrong character you should be playing Makoto

Good luck anyway I am looking forward to Central Fiction personally. No Jubei

but Nine and Makoto will keep me happy.