Doubara isn't anymore 2x years old. He should possess much more introspection, especially when he works with literature full time. At the end of the day, how is this different from any random mother, who lost their only child in an accident and has a totally non-supportive, cheating husband, asking someone if they believe the child is now in Heaven? Because going to Heaven after dying is not fundamentally any different from getting isekaied, the only difference being that going to Heaven is an official religious belief, that even billions may believe in, the other is fantasy literature.
If Doubara was a young guy, I wouldn't blame him for this, but since he's not, I do. Why the heck does he need to be 100% honestly and brutally straightforward in a case like this? This isn't about him, it's about Mio. He even realised that afterwards, but how the heck does realising it afterwards help? He should have had plenty of time to go through these things in his mind while Mio was visiting her own home, in fact even before while they were together. So, no, this was totally unnecessary and immature behavior from Doubara. I expected more from him.