New Zealand Spinach, Elder, etc.

I'm putting together my herb seed order for the year, but first I have a few questions:
1) Does New Zealand Spinach contain oxalic acid, like regular spinach?

No, it does not. True spinach belongs to the Polygonaceae, a family that has many plants that are high in oxalic acid. Other high-oxalate members of that family are sorrel, rhubarb and dock. New zealand spinach belongs to the Aizoaceae, and as far as I know few if any members of that fairly have high oxalates. You can see for yourself what new zealand spinach contains; check its chemical profile in Dr. Jim Duke's Phytochemical and Ethnobotanical Databases at www.ars-grin.gov/cgi-bin/duke/farmacy2.pl?choice=taxon&farmtax=1005

2) All the sources of elder flowers (at least that I've seen) are sambucus nigra. But in several catalogues, I can only find plants of sambucus canadensis. Would the latter have similar medicinal uses? Or should I keep looking until I find a source of seeds or plants of s. nigra?

Sambucus nigra is the European elder while S. canadensis is the American elder. Both are similar in their medicinal properties, the former being official in Britain and the latter being official in the U.S. For more information on the medicinal aspects of elder, see the entry in King's American Dispensatory: www.ibiblio.org/herbmed/eclectic/kings/sambucus.html

3) I have read the hyssop should be avoided by epileptics. I have also read that horehound and hyssop have the same active compound. So does that mean that epileptics should also avoid horehound?

There are suggestions that essential oils from some herbs can trigger epileptic seizures. It is important to note that these concerns relate to the concentrated essential oils that are extracted from these herbs and do not necessarily relate to the herbs themselves in their natural forms. Essential oils are many times more concentrated than the natural herbs – perhaps hundreds or thousands times more concentrated – so they should always be treated with great respect. Besides epileptic seizures, concentrated essential oils can trigger asthma, allergic reactions, rashes, and more.
The essential oils of fennel, hyssop, rosemary and sage have all been implicated as triggers of epileptic seizures. However, rosemary, fennel and sage, in particular, are widely used safely in their natural forms in food, which supports the notion that concentrating the oils is what elevates the risk in these herbs. And we suspect the same could be true for hyssop.
As for whether horehound could cause seizures on account of common constituents, that is possible, but we have not seen any evidence to support that. It is important to realize that even if two plants have similar active constituents it doesn't mean that they are equally likely to cause the same side effect, for the simple reason that the active constituent may not be the culprit responsible for the side effect.
Unfortunately, there is no source that we can refer you to answer your question conclusively. At this point, I don't believe that there is an adequate understanding of the risks herbs or their derivative products might present to epileptics.

4) Finally, what parts of edelweiss, forsythia, goldenrod, and heartsease are used for teas?

The medicinal parts are what we call the "herb", meaning the above ground foliage, usually at the beginnings of flowering. Except for forsythia, of which the fruits are used.

5) Finally, is there one single book that will tell me how to raise, harvest, and use a wide variety of herbs (from the basic herbs like thyme to some of the more unusual ones like forsythia)?

There is no single book that covers how to grow and use most herbs in sufficient detail, but the one that comes closest is Deni Bown's "Encyclopedia of Herbs and Their Uses" (available from Richters).