Could a fluoride varnish (concentrated form) be applied by a dentist to decrease dentin hypersensitvity and sensitive teeth due to gum recession?

Fluoride varnish. A Fluoride varnish would be an excellent treatment for the conditions you describe--either the dentist or dental hygienist can apply the varnish. It is a bit sticky and will be noticeable on your teeth for a few hours.
Yes. Fluoride varnish is recommended by the ada not only for children but adults as well. It decreases sensitivity, strengthens the enamel, and has anti bacterial properties as well. It is particularly recommended for adults with gum recession and root exposure to reduce the risk of cavities.