“Would the soup really have caused death? Was this a strange figure of speech that means something else?”
And, “Is there a modern-day explanation on why adding flour absolves it's apparent poisons? For example, a particular chemical/food reaction?"
It is essential – to understand the Bible – we make reference to context. Speculation can do his share in the work only if it is in synch with Bible context (from micro-context to the global one).
So, the context of 2 Kings 4:38-41 suggests us that that account has no relation with a meliorative food technique performed by Elisha, but, instead, it enhances the God-driven role of the prophet in behalf the people of Israel.
“What exactly was this wild gourd?”
As regards the particular kind of gourd indicated by פקעת (2 Kin 4:39) is interesting that André de Mol in his ISA (Interlinear Scripture Analyzer) translates the term with ‘colocynth gourds’. Can it be a possibility? More than that. Going by looks this plant seeems (to a not trained eye) an innocuos gourd.
But, read now what Wikipedia (in this case the article contains plain references) said of this plant (in this particular case, Citrullus colocynthis).
“Ingestion may cause irritation of the mucous membranes in the gastrointestinal tract, bloody diarrhea, or kidney damage. The ill effects are caused by cytotoxic and antimitotic cucurbitacins. The active substances pass into urine and breast milk and can lead to abortions in pregnant women. Other symptoms of poisoning include ulcers, wall perforation, peritonitis, kidney bleeding and bladder inflammation. Often brain hyperemia, delirium and collapse occur. Death may occur as a result of respiratory arrest. Cross-fertilization of colocynth with watermelon or zucchini sometimes leads to cases of poisoning. Cases of poisoning have also been observed in animals that consumed the fruits. The intake of just 3 g of colocynth and be fatal. Exposure of the skin to the active substances may cause blistering” (Wink, Michael; Ben-Erik van Wyk; Coralie Wink (2008). Handbuch der giftigen und psychoaktiven Pflanzen. ISBN 978-3-8047-2425-9)
The dangerousness of this plant is enhanced by the – as we read in the exerption above – possibility of a cross-fertilization of innocuous and healthy watermelon or zucchini. At this point it should not be difficult to understand how people can take a poisonous plant instead of a safe kind of pumpkin. The Bible account itself says that this occurred “because they did not know it (the plant)” [ידעו לא¯כי]. Sometimes knowledge makes a difference between life and death.
“Did putting flour in actually do anything? Or was it a "placebo effect?”, and “Or were the men merely being whiny, superstitious, picky eaters?”
The Elisha's use of flour must be situated in a different frame compared to the frame supposed by Rlb.USA.
Often, inside the Bible miracles accounts, we see some performed actions (along with some utilized objects) that are not necessary to the purpose of the miracle itself.
For a couple of examples, none of the following objects triggered the miracle effectiveness:
a) the piece of wood thrown by Elisha into the water (2 Kin 6:6) before the axhead was floating on the water;
b) the soot tossed toward heaven by Moses (Exo 9:10) before the boils were breaking upon the Egyptians.
The point is: the miracles cited were not performed through those objects but only through the spirit of IEUE God.
Why, so, those symbolical actions? Evidently, they worked as an illustrative device – a demonstration - that prepared bystanders to grasp what the prophet was to do.
Applying this concept to the examples cited, as if the prophets were saying to bystanders:
(a) “See! Now the axhead will float like this stick of wood is floating on the water!”
(b) “See! Now the black boils will stick on Egyptians like this black soot will stick around!”
Similarly, as if the flour utilized by Elisha speaks for him: “See! Now the poisonous soup will become healthy like a flour soup!”
Hope this will help.