Thanks for all the suggestions and info. Now here I go asking for more......
I think I will start out with an ARC AAA. I have put off getting one for far too long..... She should also then be able to change batteries herself. The Infinity is tempting because of the larger diameter, and battery life, but the loss of brightness is not desireable. (may get her both, let her decide which one I get back...).
How does the ARC twist-on required minimum force compare to the baton light's? ARC is .45" diameter, batonlite is 11/16(.69"). The arc would require her to grasp a short head less than 1/2" diameter to turn against the rest of the body and the entire keyring, while the batonlite has about 5 inches of 50% larger diameter to twist against the short tail that while bigger around than the ARC, may be hard to grasp because it is amongst a keyring that can freely swivel around it. Suddenly this does not sound easier. Am I thinking about this correctly??? How long is the tail twist section?
(I can play with the ARC for a bit and break it in for her. hehe.)
Is the ARC o-ring really needed for head retention? will weakening/removing the ring increase the probability of spontaneous disassembly? Could a "string" be "taped" to the head and body that would allow enough twist to turn on, but stop twisting beyond that point to keep the parts together? How many degrees of twist are required to go from firmly off to definitely on?
Again, thanks for all the great help. Glad I am not a cat.