For me, one of the most wonderous things about dogs is their ability to bond with multiple humans. Each time, the one they are with is the best ever, yet they mostly break and remake bonds with seeming ease.
Months 6-12 involve a lot of change for a dog and their experiences during this time will influence their adult selves. So, in leaving yours with MIL I would expect:
- him to have developed inherent character traits you won't have seen in him before you left
- him to have developed behaviour traits as a result of your MIL's environment and training
As a result, the dog you get back is likely to be a bit different from the one you left. As Battendog at 12 months was quite different to Battendog at 6 months. Though some things remained constant.
In reality, this might mean he develops some traits you don't like and will spend the rest of his days figuring out if they were just part of who he is or a product of being with MIL.
In my mind, you'll have to just accept that and deal with whatever you come back to. Of course, you might also come back to a happy, well trained, well adjusted dog having only done 50% of the work 
I wouldn't leave Battendog for 6 months but that's because I would miss him. I could well imagine him in a good home (e.g. with family), having a blast and not really missing me one bit. That's dogs for you! I personally think the loyal and unconditional love myth is the biggest bs ever
. My dogs have never been that loyal that they don't form good relationships with others and I am fairly sure only love me based on how nice, comforting, fun and interesting I am to be around. As it should be.
So I don't really think the bonding thing makes it a no-goer. Might be worth settling him into it gently, perhaps spending increasing time with MIL before you go? And then the same when you get back to transition him back to you.
Might also be worth MIL seeing if there are other children about the same ages as yours that can spend time with him while you are gone - so he remains comfortable with children. If that's possible.
p.s. no way would I go through the upheaval of moving just for six months but each to their own 