Let's say it goes all the way to CA Supreme Court and WT loses there. Can WT then try taking it to the US Supreme Court?
Yes, although they cannot appeal the special relationship/duty to warn issue to the US Supreme Court, which is probably the issue that the CA appeal court will be most interested in (and probably their strongest argument on appeal). Since that relates only to state law, the Supreme Court cannot hear that issue. Here is a link to the jwleaks post of the WTS appeal brief, which outlines the six grounds for appeal raised by WTS. It appears to me that in theory, four of the other six arguments aside from the duty to warn issue could be appealed to the US Supreme Court.
Technically, they could ask the US Supreme Court to hear the case even if the CA Supreme Court declines to hear the appeal.