Yes, activate with water, and then just use enough apple juice to lightly flavor the water and mask the taste. Or do the baking soda technique.
As seniors our immune systems are not as strong, so we're more likely to pick up "bugs" -
Yes, you can activate with 5 drops of apple cider vinegar or lemon juice, instead of the citric acid.
I've not experimented, but if you wish to use less citric acid, I would allow the mixture to activate longer - up to a minute. Look at the color of the traditional activated product - it's a dark yellow - if you use less citric acid, then let it activate long enough to get to the same color. Andreas Kalcker is doing work now with a 33% citric acid solution, and his directions are to activate it until it turns that dark yellow.
The problem (not that it's a 'bad' problem) is that they pretty much know how many ppms and so on are in the 50% activated 20 seconds. We don't have that same kind of information for all different %s of citric acid and activation times, across the board. But it certainly can be done. You will be "close enough" and since you're doing maintenance only, the small level of uncertainty is not going to be an issue.