Davide wrote "Hope that helps."
YES, it does !
1) Type of interval --- in hindsight, I should have identified my main concern was an assurance that I could omit providing a "type error" message because the system would always do it for me.
2) Your comments lead me to consider writing another problem with a non-monotonic function where the point estimate in second part is required to be in the interval answer for the first part.
3) I suspect the anomalies involved relative tolerance issues. They have not recurred after I added the following line.
Context()->flags->set( tolerance => 0.0000001 , tolType => 'absolute');
Intermediate Value theorem, custom checker
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