note: i'm not an expert, and have very little experience.
1. depend on the size of the trees, i'd guess that non-tropic, adult trees can be satisfied with water once in few days.
just note that watering frequency influences directly the blossom/ripening cycle.
2. depends on your climate - in many places, air moisture condences during the night, enough for catching it with large and rough plastic nets, like this:
you need to hang it vertically , so that collected water is dripping down, where you can collect it or direct the net toward the targets.
these need to be as tall and as wide as possible
3. it's a question of what-costs-more, manpower vs. equipment. for my needs, it cost more to water it manually than setting a dripping pipe system.
i also was lucky to find a used dripping pipe once - but 2nd hand is far from perfect since the holes do not match my plants, and all i could do was closing few wholes with a duct tape - which was better than nothing.
another option is finding / getting 2nd hand all sorts of used pipes, even half-open, and loosely connect them to a single location, so that watering takes much less time.
open channels seem to me like requiring much more maintenance than plastic pipes, due to algae, litter, dead leaves, sediment, small land slides, etc.
can i ask in which part of the world are you?