Hello my friend! I am a guitar teacher and have had arthritis since I was a teenager. However, I have never heard of someone getting arthritis from playing guitar. Many times it is a dietary deficiency or DNA. A few things that I have found that it helped my joints are vitamin D, flax oil or fish oil, and staying away from meat and dairy as much as possible. Animal products are inflammatory and will make arthritis worse. Eat your veggies!
Ryan, typically your fingers will do what you're saying if you are new to guitar. However if you practice sloppy technique over the years this will develop into habit. Check out:
I have many other fingerpicking videos on YouTube as well.
When learning how to play slide guitar, take your time! Work on the technique slowly and methodically. Slide technique is very tricky and there several components to it. Make sure that any string that you don't want sounding, is muted.
I teach and play guitar for a living. I am always telling my students that when they have an issue with something in their playing they have to slow it way down and consciously allow their brain to wrap around the specific subject that they're having a problem with. In your case of tensing up your wrist, you must think slow in order to play fast. Forget about playing fast and concentrate on playing accurately. Accuracy ultimately equals speed. However, if you try to be fast you will tense up and your accuracy will be off. No good in having speed if accuracy is not there. So, slow it down and practice with a relaxed wrist.
There are many different strumming patterns in that song. It would save you all a lot more time to understand how where and when to strum on ANY song. This way your ear will tell you immediately what to do. I have several videos on YouTube to teach you how to do this quickly. Check it out:
For speed picking there are two basic schools of thought. Strict alternate picking and economy picking. Yngwie Malmsteen would be an example of a player that uses strict alternate picking. Alternate picking is always down up down up. Economy picking is based on what direction your next pick motion is. Steve Vai does both methods, but is REALLY good at economy picking. Difficult to explain in a thread. Much easier to understand in video. YouTube search for these two items.