I’ve testified in a variety of courts and administrative hearings. As a witness, I don’t recall ever having the option to pick and choose what questions I would answer. Sure, the judge could rule on the relevance or the or the form of a question. Usually only after the question was asked and one of the attorneys involved raised an objection.
According to the article the so-called whistleblower has agreed to answer questions posed by Republicans only in writing. This, before the question is asked. https://www.breitbart.com/politics/2019/11/04/lawyer-ukraine-whistleblower-willing-to-answer-gop-questions-in-writing/
This guy is dictating how he will present his testimony. Democrats can pose questions without restriction. Republicans can not.
This a case of the ends justifying the means. The process is bastardized, to achieve the desired result. If the end is still not met, then it is necessary to adjust the process until the end is met. If the end is so egregious that it will never become the desired result, that’s okay. A continuous loop of charges and inquiries, with no end in sight is not a bad secondary result.