Are my wife's clear visions, which come true and increase with her commitment to prayer, considered a karamah (a miraculous gift from Allah), and is it permissible for her to continue treating and advising people based on them, or should she stop?

The vision of a believer may come true and is considered a part of prophecy; it is among the glad tidings if it is righteous and from Allah. There is no harm for one known to have truthful visions to advise his Muslim brother, for Allah may reveal to some of His servants what is hidden from others. This is part of the inspiration or miracles (karamat) that Allah grants to His Awliya' (saints/devout servants). An indication of inspiration is that Omar ibn Al-Khattab, may Allah be pleased with him, was a muhaddath (one inspired to speak truth), just as Satan has an instigation towards evil and the angel has an instigation towards good.