Attaching a screenshot to illustrate how you can receive any reply mail (in any post) without opting for "all mails" in the group. I follow this practice and usually it doesn't fail.
If you open any post in the group site you would notice an icon of 'envelope', at the top right corner (if you haven't already opted for "all mails"), by clicking this icon it will turn dark-shaded and you will receive all mails relating to that post, even if those mails are not directed to you.
Thank you once again.
Regards,
surajit