Is a marriage that took place without witnesses, a guardian (wali), a judge, or documentation, and with a very small dowry, considered legally valid (sahih shar'an), and what are the implications concerning pregnancy?

Legitimate marriage has conditions and pillars without which it is not valid, among them are the guardian and witnesses. What occurred between you was not a marriage but an illicit relationship. The obligation is to repent to Allah by abandoning the sin, regretting it, and resolving not to return to it, while concealing oneself. If you both believed this marriage to be valid, then the pregnancy is attributed to you.