The danger of mixing 5-htp and antidepressants is due to serotonin syndrome, which indeed can be fatal, but is still really a matter of dosage. They can be combined safely, but yes, it can be extremely dangerous if one does not know what what they are doing. 5-htip is very potent, approximatly 29 times more so than L-tryptophan, which might be a safer alternative.
Indeed the study of how anitdepressants and neurotransmitters work as to managing our emotions is still not well known. Some time ago, certain B vitamins were even prescribed for depression, and several of these do in fact affect neurotransmitters. Currently the thory is that depression stems from depleted neurotransmitters, but some studies show this not always to be the case. The relativly new antidepressant tianeptine actually enhances the reuptake of serotonin, rather than inhibiting it, despte this in studies, it still shows promising results toward reducing depression in some patients.
The simple fact, is of course that neurochemistry varies greatly between individuals and depression and anxiety, while often appearing to be a symptom of biochemical variations, most often actually stems from real world problems that people face. What works well for one person can be dangerous to another, and it is quite possible for someone to suffer from depression or anxiety, despite their actual neurotransmitter levels.