What's in humic acid? Is it actually a mixture of several organic acids? Does it contain EDTA or other chelation agents? Are there other organic chelation agents in humus?

Humic acids are a complex mixture of partially "decomposed" and otherwise transformed organic materials.  The chemistry of their formation is quite complex, and freshwater humic acids can come from a variety of cources, most of which are on land (decomposing terrestrial vegetation.)  These substances wash into lakes and rivers, undergoing further transofrmations along the way, and ultimately into the ocean.  Most but certainly not all of the marine humic acids also ultimately have their origin on land.  Almost all of the lignins found in marine environments originate on land.

There are several subclasses of humic acids, (tannins, lignins, fulvic acids) which are partially "resolvable" based on some fairly simple physicochemical criteria, but the criteria for these separations is primarily the convenience of the methods used, and to some degree elucidating their origins, as opposed to their functional impact on aquatic ecosystems, so I won't go into the different classes or how they are distinguished.  They all tint the water yellow and they all bind cations.

All share these characteristics:

A substantial fraction of the mass of the humic acids is in carboxylic acid functional groups, which endow these molecules with the ability to chelate positively charged multivalent ions (Mg++, Ca++, Fe++, most other "trace elements" of value to plants, as well as other ions that have no positive biological role, such as Cd++ and Pb++.) This chelation of ions is probably the most important role of huic acids with respect to living systems.  By chelating the ions, they faciliate the uptake of these ions by several mechanisms, one of which is prevneting their precipitation,
another seems to be a direct and positive influence on their bioavailablity.  Many organisms can expliot dissolved organics to some degree if they are present, and humic acids may be taken up by this mechanism.  Another paradoxical effect of humic acids is the detoxification of heavy metals.  One might expect them to be made More, not less toxic by humic acids, but the studies that I've seen seem to
indicate a detoxifying effect.

Humic acids also have a smaller fraction of phenolic functional groups, which can be detected various chemical methods.

They are derived from peptide, lipid and carbohydrate precursors.  Their formation and diagensis is partially mediated by aquatic bacteria and enzymes.

They are all complex chemically, polydisperse (having different composition and molecular weights) and eventually hit a diagenic state where it is difficult to change them further.

Terrestrial humic acids tend to be more "aromatic" in nature (having more benzene- and phenol-like components) while marine humic acids tend to be more aliphatic in nature.  The distributions seem to be overlapping, so this is a difference in degree.  There are certainly aliphatic (grease, oil, fat) like components in terrestrial humic acids, as there are aromatic components in marine humic acids (since I've easured phenolic compounds in marine aquaria in the past.)

I wrote an article for Aquarium Frontiers some time ago rgarding the effect of humic acids (and their clearance from the system) on visible and ultravioloet light penetration into aquaria.  HUmic acids are expected to have a similar role on light transmission in freshwater planted tanks.   It is not clear to me how sensitive your plants are to UV radiation, or how much UV output typefies the light sources you use. 
Dana Riddle will have an article in the coming issue of AF which describes the amount of UV emission from some (not all) light sources used in marine aquaria, which some of you may be using as well.  We have a freshwater planted tank in lab, I ought to do an activated carbon treatment on it at some point.  We are using incandescent light on it (wasn't my decision, I would have put metal halide pendants on it had it been my call) so I'm not likely to see any rapid and profound biological effects, because the UV emission in this case is virtually nonexistant.

You asked about EDTA.  EDTA is a synthetic compound,  ethylenediaminetetraacetic acid.  It has four COO- groups on a very short aliphatic backbone.  It is a potent chelating agent, and can mimic to some degree the chelation of metal ions by aquatic humic acids.  While I don't know that EDTA has ever been isolated from natural humic acids, it might be present in vanishingly low concentrations.  The important part is that it is to some degree a "look-alike" and "function-alike" of humic acids.

Humic acids or molecules with similar properties have probably always been around, so life has been "safe" in being somewhat dependent on them.   Functionally similar molecules may well have been produced abiotically before life existed, and the "goop" that is found when you zap energy through a reducing mixture of gases like that found in the early earth's atmosphere has characteristics reminiscent of humic acids. 

Most trace element suppliments contain some sort of chelating agent (EDTA or functional equivalent) to help stabilize the transition metal ions in solution.  Peat is a rich source of humic acids, as are decaying driftwood, and the diagenisis of fish poo in the substrate of your aquaria.  ;)  "Blackwater extract" seems to be mainly humic acids.

Humic acids are removed by activated carbon.  There are clear implications for fate of trace elements in aquaria, perhaps Especially freshwater aquaria, in that statement.