"Interesting. Why do Saphs not get FC line but Plats do?"

Business class check in is a Sapphire Benefit on OW carriers. AA allows its iwn Platinum members to use 1st class check in though. Other OW carriers have benefits that apply to their own members over the OW benefits. e.g. BA Gold can use BA lounges regardless of which airline they are travelling on but can only use other OW lounges if travelling on a OW carrier

"Are there any other disparate benefits for Plats vs. Saphs? Can Plats use FC lines on non-AA OW carriers..and what about other Saphs?"

Platinum members, being sapphire status, are entitled to Business class check in on other OW carriers . Only Executive Platinum members are entitled to 1st class check in on other OW carriers.

Dave