How can chicken offal be cooked to be as tender as possible?
With every (edible) part of an animal, you should consider the former duty of the piece of meat at hand. This will influence the internal structure and thus the best method to prepare it.

So throwing all chicken offal into one pot at the same time is probably not the wisest idea.

Lets "dissect" what we are dealing with here:


Liver and kidneys
Basically no muscle fibers, just a very tender large gland. Keep in mind that there are no fibrous parts that "melt down" during a long cooking process, just tender stuff that gets very hard and dry when overcooked. So short is the key here - a quick sear in a hot pan and you are good to go. Depending on your personal preference, slightly pink or "just not pink any more" would be a good point to serve.
Note that especially for organs from larger animals careful preparation is important: remove all hard parts like "vessels" and excess fat and wash kidneys well or soak them for half an hour to an hour. Some suggest soaking in milk.
Hearts and stomachs
Just the opposite - a hardworking muscle, garnished with a good amount of collagen. Here, you need time to break down the collagen. When you heat heart or gizzard, first the muscle parts toughen up, then the collagen breaks down and the piece gets tender again. Braising is a common method to acchieve good results.
For chicken hearts, sources vary between 15 and 25 minutes braising after roasting/frying them in your pan. It may depend on the quality of meat at hand, where your personal "just right" is. Larger animals' hearts may need much longer, a very old cook book of mine suggests 1.5 hours for beef hearts.
Chicken gizzards are even tougher than hearts: did you notice the white lump of collagenous fibers on each side when cleaning them? Give it time to get tender. It may take from thirty minutes to over an hour - sources vary.