I have used fertilizer for very long time and I know that urea is for fast response to growth and npk is for good seed and both can mixed up together but the urea will be less in quantity
Nitrogen (N) can be lost from the soil through three ways namely: denitrification, leaching and surface volatilization (please take note). Also, six major factors determine nitrate-N levels in soils which are: soil clay content, organic matter content, pH, temperature bulk density, and rainfall. It is best to prevent volatilization if you burry the fertilizer a bit (not too close) to the root zone to prevent its loss in a gaseous form through volatilization. It is also good if you do not apply the fertilizer only to have a heavy downpour which washes the fertilizer away from the root zone. Timing of the weather prior to its application is key especially if the rain shall be mild to help dissolve the fertilizer and not to leach it so much from the root zone. You also mentioned exothermic reaction during the dissolution of the fertilizer which kills the crops. Rather, it is osmotic difference (plasmolysis, where the fruids from the plant move out to the fertilizer (a salt) in an attempt to reduce its concentration before it is taken up through Cation Exchange Capacity. All the best. Thanks