I have no experience with TENS systems. They are usually used for pain relief. If they can relieve muscle tension, that is great.

Your neuro is odd. My neuro told me to take anti-inflammatories like ibuprofen and aspirin. Omega 3 fish oil also thins the blood. You are not at risk of a brain bleed so blood thinning should not be an issue. btw, I've been taking enteric aspirin daily for 25 years for its anti-inflammatory value. I bleed like crazy when I cut myself that is not a concern.

I'd like to know what the long term problems can be.

It has been long enough since your injury that any risk of a bleed should be long passed.

Constantly having muscles in contraction is just a constant spasm. Maybe you need to learn some muscle relaxing techniques. There is a concussion symptom called postural rigidity. I had it real bad. I held my upper body muscles in contraction for years. I had to get new clothes because I developed lats and deltoids that I never had before. I have to learn to release the muscle tension.

The subtle neck injuries that plague many of us do not show up on imaging or manipulation. They just cause inflammation that affects the nerves and blood vessels and blood flow.