I am not sure who I feel more sorry for- you or this little boy!!
I also don't understand how parents can raise a child one way for an entire year, and then go back to work and think he/she will magically adjust to an environment that is vastly different than what they have seen so far.
I would be okay with the sleep-sack, if it fit him properly. He should have room to stretch his legs straight. I would not be swaddling a one year old.
And I would make sure mom and dad understand the difference between home and daycare. You will not be carrying him everywhere. You will not be in his sight for every minute of all day. And so on.
I think he will adjust eventually. Almost all kids do. But as for a timeline- that is so hard to say.
Wishing you good luck!!