#1. I'd be careful about over-working the flexors(flexors close the fingers, extensors open the fingers). There is a natural imbalance between the muscles of the forearms and fingers. The flexors are naturally stronger than the extensors. It would be wise to work both sets so that strength would be developed equally between both sets of muscles. Otherwise one may run into one or more injuries/syndromes (from carpal tunnel & repetitive stress injury to tennis/golfers elbow & even bone structure changes that may become issues such as osteoarthritis).
#2. There is a silly putty like material used by Occupational Therapists or Physical Therapists in recovering hand & grip strength. It may be something to look into.
As one builds strength in one area, other areas are going to have to be strengthened also. If not, it may lead to an imbalance which may lead to injury. Example, grip strength without working extensors or wrist & elbow stability. Everything in the body is connected in one fashion or another.