@goodhunter I think @Eals may have it right about them being parked. If they were driving, how could she give him head in that position from the passenger seat?
Page 19:
"Is Chris-san alright?"
"On the surface, she looks calm and ready to drop bombs, but she keeps on forgetting..."
"Mom's spaghetti?"
"Not yet. We were planning on keeping her here a little longer to find out, but that idea was shot down."
"Since it's just a child's word, let's just a agree with him."
1) That backfired. 2) Or not if the child was right. Truth is that they don't know how and/or who summoned Japan to that world. If it was the Sun God... then it definitely would be because of their flag.
For those wanting answers to certain things, look at the comments about the manga rushing like crazy through the LN, and conclude that the manga skipped that.