Maybe best not to wean him while you are away, but why not start now, that way by the time you are ready to go he would almost be there and it wouldn't be such a dramatic transition for all involved.

Also if MIL hasn't figured it out... bugger her, not her bubby and you can feed till your hearts content. If you have to tell her for when you go away then do, but if you don't feel it's neccesary then don't.

With the shifting to cows milk. If DS has not been switched before hand he is likely just to spit it at MIL (which may or may not be the desired effect). Maybe try halving breast milk with formula, alternately halve breast milk with cows for a while slowly increasing the amount of cows till it's all swapped over.