I think yes, email interview is the optimal solution for remote communication. Especially in the current situation of the SARS-CoV-2 coronavirus pandemic (causing Covid-19 disease). In addition, if you need real-time conversation, you can also use online messengers.
Given the current circumstances, i would think its ok. However, email responses might not be as 'deep' or 'rich', so data quality might be compromised. Try some online communication tools. But yes, as a last resort you can use email.
Only if this is the only alternative. For example, you want to interview someone who lives in a different timezone or an expert who expressly says they prefer to answer via email.
I think that the online interview is entirely acceptable in the current situation and will be used more and more in the future. In addition to e-mail, various social media applications can be used. Whether it is an online or offline interview, we should adhere to high standards and professional implementation.