Hi Tan!
I know how you feel. I work 5 days, MJ is with mum for 2 and daycare for 3.
I can tell you that in my experience, for all the threats and taunts of giving him this and that, she's not done it once. When I am there its a different story and she will try to sneak him stuff, but I know from his behaviour and the fact that his skin hasn't broken out that she isn't giving him junk when we arent there.

As far as the attachment stuff goes, it differs widely from one child to the next (you probably know from school some kids are more prone to this than others) but I haven't noticed it. Sure he is very comfortable with his carers at creche, and very close to my mum and dad, but I don't feel that its affected his relationship with us IYKWIM? Especially when Grandma or Grandad says no! ha ha