Though this was posted a while back if you are still concerned I think I can be of some help.
While I do think internet medical advice is probably not the best idea ever I am a physiotherapy (physical therapy in some countries) student and use hook grip often myself. First of all you are probably thinking along the right lines in that nerve involvement is likely with “nerve” symptoms like altered/abnormal sensation (tingling/pins and needles + numbness) as opposed to musculoskeletal symptoms.
Nerves, being physical structures, can be impinged, trapped, poked and prodded. Physical irritation to a nerve can give rise to various symptoms. A common example is the funny bone where the ulnar nerve running by your elbow is close to the surface so a well placed blow with the elbow bent will result in a electric shock-like sensation. In your hand/thumb there a also nerves running and it’d be my guess that the sustained pressure from the heavy loads irritated the nerve itself or structures which it runs through/beside resulting in your symptoms. It seems that no permanent damage has been done, the nerve probably just being irritated, however you’ll probably have to make some modifications to your training in light of this.
The easiest fix would be taking a look at and modifying how you use hook grip.
As you described 10 reps with 5 second eccentrics is a long time for your tissues/nerves to be under load and frankly the hookgrip is probably not meant to be used that way. Weightlifters hook every opportunity they get but these sets are usually low reps. Powerlifters (some at least) hook heavy but usually for low reps. Some don’t even hook over three reps and personally my thumbs is start tearing up, especially without taping, after 6 or so.
What are your goals? Why do you use hook grip? Are you a bodybuilder who wants to avoid muscle imbalance with mixed? Conscious of increased injury risk with mixed grip? A powerlifter who wants to pull max weight? A sumo puller who uses hook grip to prevent your supinated hand snagging your thigh? Do you need to hook for high reps or even reps?
Coming from the perspective of a powerlifting bodybuilder who cheats/sumo and hook grips I’ve found pulling hook to be best reserved for lower rep sets. Everything else is straps. This is for a couple of reasons but mostly being a little wuss/ saving my hands for when it really matters. Also I’ve found hook grip to feel the same after a certain weight so you don’t need to smash your thumbs to get used to higher percentages.
Also keeping hook grip fresh is as easy as using it. Sparingly is enough though, so if you get in a few reps, even just warm ups, you’ll be acclimated to hook grip and thus able to pull it out for competition weights at any time.
Straps will enable you to get in the work without worrying about your skin or nerves or whatever. I find the feel very similar between straps and hook as both kinda aren’t using your grip but some other means to hold the bar like friction for straps and the hook for thumb hook grip.
Perhaps a small change in your hook grip technique will help you out. While I can’t say to have had similar issues to you a change to my hook grip helped me when I was always putting pressure on a bone when pulling. Squeezing harder or softer, wrapping more or less finger and changing wrist position may all help and even give you a better hook.
This is not medical advice however and if you are really concerned, maybe if the issue seems to be progressing/worsening or occurs outside of using hook grip, you should see a professional e.g. doctor or physiotherapist for a thorough assessment.