Can one return to her husband if his statement "If you send the message, he will divorce me" was a threat and he did not intend divorce by it, and it was issued in writing, not verbally, knowing that this is the second divorce?

The first innovative (bid‘ah) repudiation is effective. As for the second repudiation, if it was in writing, then the husband’s intention is considered: if he intended to effect the repudiation, it is effective; otherwise, it is not. If he neither uttered the repudiation nor intended it, then the woman is still under his marital bond, considering that he took her back after the first repudiation.