Is it forbidden to agree with a young man to continue gradual communication with the intention of completely severing the relationship after a certain period of time so that he can marry her, knowing that there is a prohibition in this temporary communication?

The questioner made two mistakes: establishing a forbidden emotional relationship and assuming that this method reveals a person's true nature. Good intentions do not justify a bad deed, and often a person's outward appearance contradicts their reality. Therefore, the relationship must be severed immediately, and inquiries about the person should be made through trustworthy individuals and acquaintances, followed by seeking guidance through Istikhara (prayer for guidance). It is also advisable not to prolong the engagement period and to expedite the marriage through the easiest means.