Kaylin's Great Warping and Alternate Form have the same base with regards to hit points, so the same logic that changed your max HP when you became a dragon would change it again whenever Alternate Form changes your constitution.
About darkvision, the full details of the
Alternate Form ability state "The creature retains the special qualities of its original form. It does not gain any special qualities of its new form." Darkvision is a special quality and wyrmling silver dragon is your "original form" for Alternate Form, so you keep the dragon's 120 ft darkvision.