Upvote:4
A 2017 Christianity Today article about this very issue was written by Mark Galli shortly after his own Karl Barth biography was published. As curiousdannii suspected, he did use dialectical tension to justify it, part of the discussion by Christiane Tietz in Theology Today, of the personal letters Barth wrote to von Kirschbaum from 1925–1935.
A lot more details, analysis, and paraphrases from Tiez's article can be found in a very well-written blog article here. The blog article in turn highly recommends a "book review" of Charlotte von Kirschbaum and Karl Barth, 1998 which actually not so much a book review since 85% of it is about relevant external history of their relationship.