When to use "expectative" instead of "expectation"?
The best advice is 'Don't use it'. Your first example is a technical term in canon law, and there are various obsolete or obsolescent contexts where it can be used; but most English-speakers think (correctly), that the adjective from 'expect' is 'expectant' (for the person expecting) or 'expected'.
Expectative is an adjective not commonly used and expectation is a noun. For example,
The team has expectations of winning this time.
This article is about the team's expectative victory.