Is the separation of a committed young man from his fiancée a few days before the marriage, despite his devotion and love for her, considered a trial for purification, a دفع لسوء (warding off evil), or for another reason?

The result of Istikhara (guidance-seeking prayer) is either success or its absence. If the marriage does not take place, it is better for the one who sought guidance, in fulfillment of the Almighty's saying: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." It might be a trial through which Allah raises one's ranks and expiates sins, as the Almighty said: "And We test you with evil and with good as a trial; and to Us you will be returned." A Muslim should entrust their affairs to Allah and supplicate, for Allah does not burden His servants. Whoever is not successful in one matter should seek another, in fulfillment of the Almighty's saying: "And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided."