Great list. Excuse me if I duplicate, but here are a few additions:
Many from the Engaging News Project by Stroud et al test how to improve comments sections (https://engagingnewsproject.org/research/)
e.g., Stroud, N. J., Scacco, J. M., Muddiman, A., & Curry, A. L. (2015). Changing deliberative norms on news organizations’ Facebook sites. Journal of Computer‐Mediated Communication, 20(2), 188-203.
There’s also some work on how comments influence perceptions of news content, e.g.,
Anderson, Ashley A., et al. “The “nasty effect:” online incivility and risk perceptions of emerging technologies.” Journal of Computer‐Mediated Communication 19.3 (2014): 373-387.
Kümpel, A. S., & Springer, N. (2016). Commenting quality: Effects of user comments on perceptions of journalistic quality. Studies in Communication|Media, 5(3), 353-366.
Prochazka, F., Weber, P., & Schweiger, W. (2016). Effects of civility and reasoning in user comments on perceived journalistic quality. Journalism Studies, 1-17.
And the reverse, how other content influences perceptions of comments and commenters (full disclosure, this is my work; hopefully it has some relevance here):
Veenstra, A. S., Park, C. S., Lyons, B. A., Kang, C. S., & Iyer, N. (2015). Intramedium interaction and the third-person effect: how partisans respond to youtube ads and comments. Cyberpsychology, Behavior, and Social Networking, 18(7), 406-410.
Lyons, B. A., & Veenstra, A. S. (2016). How (Not) to Talk on Twitter: Effects of Politicians’ Tweets on Perceptions of the Twitter Environment. Cyberpsychology, Behavior, and Social Networking, 19(1), 8-15.
Muñiz, C., Alvídrez, S., & Téllez, N. (2015). European Public Sphere| Shaping the Online Public Debate. The Relationship between the News Framing of the Expropriation of YPF and Readers’ Comments. International Journal of Communication, 9, 19.