Please forgive me if I am repeating anything that was already discussed, I skimmed through the previous posts, but didn't read every word.
1) Flax seed contains compounds that slow the growth of pullets/cockerels, but these can be reduced or removed by heating in a microwave or oven. (if you want a heating pad, put some flax seeds in a sock, tie the end in a knot and microwave it. After you've used it for a bit, feed the seeds to your chickens and make a new one).
2) the reason not to simply eat flax your self is that flax seed (and other plant sources of Omega oils) contain ALA, which is not necessarily a health benefit in itself, but can be converted to DHA and EPA by animals, including chickens and fish.
3) It may be that feeding high levels of flax causes eggs that taste fishy. Studies have found this with canola meal, especially in RIRs and related hybrids.I haven't looked closely into the research on this.
I feed a layer pellet that includes flax, and also sometimes give treats of whole flax seeds (microwaved).
This is important to me, as I have two growing daughters who are vegetarians, but eat our eggs. There is very little DHA and EPA in a vegetarian diet, although the human body can synthesize limited quantities of these from plant based ALA.