Hmm...the last question is good. I'm not sure I would accept the result of one study, but I would acknowledge it. (I still need to look into it more, but the current state of porn effects research seems to draw conflicting conclusions. If there were many psychological and sociological studies on hentai and loneliness, I would be more willing to accept it.) If I'm allowed to get a bit academic for a minute, there's also the issue of disciplinary conflict here: psychological research might draw one conclusion while anthropological research might draw another. These two disciplinary have different ways of understanding the world. Part of the reason I'm doing this degree as opposed to a traditional one is so that I can learn how to resolve these conflicts between academic disciplines.
That being said, although there are clearly examples like incels that would seem to suggest negative social impacts of hentai, I believe that for most otaku, sexuality is highly social. Anthopologist Ian Condry discusses the "soul of anime," which he believes is rooted in sociality and collaborative creativity. You can pretty clearly see this with doujinshi and Comiket: hundreds of thousands of people coming together to create and consume hentai. Some people will buy a doujin from a circle just to keep a social connection, and circles themselves are groups of friends.
In addition, the weddings some otaku have with 2D characters aren't necessarily for the sake of the character: it's for the friends. Weddings are highly symbolic affairs, and involve a lot of people coming together. By marrying a a fictional character, otaku can strengthen real-life bonds.