Is the mixing order important when substituting white sugar + molasses for brown sugar?
As you probably know, the creaming step is important.

Sugar crystals cut up the fat and make a lot of air pockets.

If liquid is added early then the sugars dissolve and creaming doesn't happen.
If there is too little sugar it is less effective.
If there is too much sugar the fat is reduced to a crumbly mass and is not fluffy.

Your recipe calls for a certain amount of white and brown sugar to be creamed and then more molasses to be added with the wet ingredients.

The safest method would be to thoroughly mix your white sugar and the white sugar + molasses to approximate the original recipe and then cream- adding the additional molasses later. Beyond this substitution you are creating your own recipe and it will require some experimentation. 

Try adding all the molasses in the initial mix and see if that doesn't dissolve your sugar too much.
Try leaving all the molasses until after creaming.
In both cases- the creaming is successful if the butter is creamy and fluffy. Not a batter and not crumbly sugar.

Obviously I would do these experiments on a smaller portion and not risk a huge batch.

I doubt anyone could tell you exactly how this will turn out without experimenting. I've never seen a table that lists the water concentrations at which creaming is effective.