"I have heard that whiplash can cause concussions."

You heard wrong. Whiplash and concussions are entirely different. Some people talk about cervical concussion and use the great overlap in symptoms to try to validate the idea. Whiplash can cause inflammation in the neck that can change blood flow to the brain that can cause neurological symptoms....

Amitriptyline has a good track recover at low doses before bed. It can help with headaches and insomnia.

If you are cautious about meds, you could try some over the counter supplements. I like a supplement I get on Amazon by Deal Supplement called 5-HTP. It is a combination of 5-HTP, L-Theanine, GABA, and L-Tryptophan in a single capsule. You could try taking one capsule a day for a week or two then add a second if you are starting to see improvement. Only Deal Supplement makes it in this combination. It is a good price compared to taking the individual components. I stopped taking a powerful high dose SSRI after 12 years and replaced it with this combination. I don't live with anxiety but my mind is very easily over-stimulated.

Also, get on a good vitamin supplement regimen. The sticky at the top regarding vitamins has my regimen that a doctor prescribed for me 40 years ago. It made a big difference. I've been riding in this rodeo since 1965 with 4 major new injuries along the way.

Regarding sleep. When I was suffering bad whiplash, I got my best sleep in a recliner chair. I used a loose pillow bunched up around the sides of my head so my head did not roll to the side. My whiplash was so bad that poor sleeping posture would cause inflammation that would disrupt my breathing. I had serious Central Sleep Apnea that made everything worse. I also fell asleep watching music videos and later a few selected CDs. Quality music and voices allow my brain to zone out. Dianna Krall's The Look of Love light jazz was amazing. Her voice would calm my mind and I would fall into a deep sleep.

A recliner would also help your GERD.

Gentle PT could be useful. Most PTs push for improving Range of Motion. That should not be a goal for a few months. Gentle cervical traction and vertebra mobilization can help. C-1 to Occiput (base of the skull) is often unstable and needs disciplined care. Imagine your fall. Your head was stopped by the floor but your neck wanted to keep moving toward the floor. This causes C-1 to slip out of place from where it belongs. C-1 to occiput is not a deeply faceted joint so it does not self-recenter like most other vertebra do.

Try a recliner if you can. They can often be found for free on CraigsList or FB marketplace in good condition.