Are a girl's brothers entitled to force her to accept another suitor on the pretext that the first suitor is delaying the completion of the engagement proceedings?

If the guardian of the girl agrees with you on marrying her, this is considered a commitment and an engagement, and does not require ceremonies. It appears from the biography of the Companions that they did not delay marriage, and the Hadith of Jabir indicates the completion of the matter without delay, because delay may lead to undesirable consequences. The consent of the girl's family to her marriage to you is a promise that should not be unfulfilled except for a legitimate reason, such as a long period before the marriage contract if harm occurs.