Responses

Yeah, Tamiya was smart enough to make out right in front of Satoru's house. The mom is shocked (she drops her groceries but then picks it up and walks past them, then peeks at them from the doorway)
OP was right about Satoru's mom just sulking about not knowing, but the dad didn't really care either, since he said as long as the mom's happy it's fine.
and the fact that if they ever said to the mom that the are a couple, she wouldn't be mad but rather sulk because they didnt told her sooner and the father also wouldnt get mad but disappointed at them making their ever 'happy' mother sad.