Mio is in grief, still very deep in the denial phase -grief is far more pronounced the longer and more deeply you loved, after all. She's acutely aware Taiga isn't in this world anymore, but if we were to replace "Another World" with "Heaven", is she really behaving so far out of touch with reality? There have been people who, in their grief, have looked at their dead children, those with injuries no one could survive, and get genuinely angry at doctors who didn't spend what they felt was an appropriate amount of time trying to save the deceased's life. There are plenty of people who 'go to join their loved ones in heaven' after someone close to them dies. Voluntarily or otherwise.
Throughout the entire manga, Haruji's acted with near disdain for Mio, and it's been made clear on multiple occasions that it's been that way for most of their marriage. In Ch12, Haruji himself asks, "When have we not been fighting?" in reference to an acquaintance talking about Mio having flown the coop. It's true that we're not shown how strong/weak his connection to Taiga was, and his behavior towards her may or may not be exacerbated by grief, but it's not new. Don't defend it.
In all this context, Doubara's not much different from a pastor that's giving her a slow and steady excuse to keep living, until she's able to leave her grief behind in a more healthy manner. Just because he's connecting with her outside the context of religion shouldn't undermine his efforts. He doesn't believe in what she's trying to do, but he's still genuinely trying to keep her alive, and is using his foot in the door by being someone she trusts on the topic to guide her back to wanting to live. He's not the best choice, but right now he's the only one she trusts.