What type of wood is this? The white fungus may be highly toxic to fish. Mopani wood, manzanita wood and grapewood are all known to carry toxic fungus. I'm not saying that all of this wood does, nor that all fungus is toxic--it depends upon the species of fungus which requires a microbiologist to determine--but it can be toxic and i would not use any wood that secretes fungus. I lost fish from this, and know of others too.
On the melting, this is normal for crypts. They do not like being moved. They will also melt, some species more than others, if the light intensity or spectrum changes, if GH or pH changes significantly (to the plant), nutrient supplementation changes (more or less fertilizations), temperature changes significantly, etc. They should recover, sometimes in a few days, sometimes it takes weeks. Leave the roots alone, siphon out the mush from the dissolving leaves and stems, and await new leaf growth.
On the fertilizers. Flourish Comprehensive is a complete nutrient supplement (except for oxygen, hydrogen and carbon) and on its own is sufficient to balance low to moderate light. You can dose it once or twice weekly, depending upon the specifics of the aquarium. The GH of your tap water, fish load and feeding all affect this, along with the plant species and numbers and light intensity.
Do you know the GH of the tap water? This is the prime source for the "hard" minerals like calcium and magnesium; the levels of these in Flourish is minimal because most tap water in NA has sufficient. The GH will clue us in to this.
I would be careful adding iron. There is sufficient in Flourish to balance the other nutrients, and there is iron in the Flourite substrate. Iron is a heavy metal and thus highly toxic to all life, be it plant, fish, bacteria, whatever. Excess iron will partially be taken up (to store, or as a toxin) by some plants, but as this is another unnecessary risk, I would not add iron individually. Nutrients for aquatic plants must be in a fairly specific proportion; excess of some can cause the plants to shut down assimilation of others.
Final comments on Excel. This is a product I do not recommend. It's only ingredient, apart from water, is glutaraldehyde which is a toxic disinfectant used in hospitals, embalming fluid, antifreeze, and similar. Some plants, such as Vallisneria, will melt completely and die when Excel is used. If it happens to be overdosed, other plants may be affected, and fish can be harmed and even killed. Bacteria will die too. Given the risks, I would not use it. Once there are fish in the tank, organics will accumulate in the substrate and provide a lot of CO2 naturally. Balance the light and other nutrient supplementation to this and you're all set.