What dangers/risks are there in the production of carbon subnitride?
There is no way, dicyanoacetylene can be produced at room temperature. According to the paper &quot;Syntheses of Dicyanoacetylene&quot;, decomposing 4,5-dicy ano-1,3-dithiol-2-one at 600-800°C, a 59-76% yield of dicyanoacetylene is obtained along with side-products like carbon disulfide, carbonyl sulfide and sulfur. Another scheme is pyrolyzing solid dicyanodiazomethane at 220°C, which polymerizes. The polymerised product is decomposed at 700°C to get desired product. Further, a note on the storage of dicyanoacetylene is mentioned:

Dicyanoacetylene is a hazardous material. Because of its toxicity and
high vapor pressure at room temperature, it should be handled only in
a hood. It is potentially explosive, both in the pure state and in
concentrated solutions, but its thermal stability in dilute solutions
in inert solvents is greater than previously re-ported. Pure
dicyanoacetylene turns dark slowly at room temperature, but it can be
kept almost indefinitely at Dry Ice temperature.

Dicyanoacetylene can only be kept at very low temperatures, in dry ice or liquid nitrogen, like a dewar, away from light and oxygen, as it may polymerize. Do not store it for long. Diluted solutions of dicyanoacetylene are much more stable, and can be safely kept at standard conditions.
Reference

Syntheses of dicyanoacetylene, Engelbert Ciganek and Carl G. Krespan, The Journal of Organic Chemistry 1968 33 (2), 541-544, DOI: 10.1021/jo01266a014