I think you are over-thinking this. You will see that different publications show different K-values and other data and in real-life many fittings get made in a shop and vary from the text-book fitting. Especially the more complex ones. So there is quite some variety. Imagine a Y, or bullhorn etc. Also note, that the tests are made with long duct up and down-stream. In real life you have multiple fittings close to each other and they influence each other. For design, just use the higher pressure drop to be conservative.
Also allow for some field-changes, like routing around a structural element. And your plans may show a 90°, and the contractor will use an assembly of 45° or some other deviation since the sprinkler guy decided to install his pipes before the ducts.