Could rectocele cause repeat utis? What complications could arise if it is severe and combined with uterine prolapse? Could it be source of symptoms?

Generally no. A rectocoele occurs when there is a breakdown in the supporting tissue between the rectum and vagina; causing the rectum to bulge into the vagina. Many women with a rectocoele have no symptoms, but those that do may notice a bulge at the opening of the vagina or difficulty having a bowel movement. In some cases, women have to push the bulge back into the vagina in order to have a bowel movement.
Could well be. You are very young to have a rectocele & uterine prolapse, suggestive problems resulting from pregnancy or neurogenic problem. Your problem is likely associated with inability to completely empty bladder with urination. Advise having an experienced gynecologist take care of this problem.
Uti and prolapse. A rectocele could contribute is causing some degree of urinary obstruction or if having difficulty cleaning after bm's. Prolapse in general can increase your chances of getting uti's.