I am amazed that your PIL's priest is allowing you to do this, and I imagine it is only because they are in very good standing with the church that the rules have been bent. Usually parents have to attend catechism courses prior to christenings so that they understand the vows they will be taking on behalf of their child, and doing anything (even attending church regularly) outside of your own parish is really frowned upon.
So I think you have to recognize this is not just a party that your PIL have organized, but a very important religious ceremony, that to them and their priest is a very big deal. I imagine that they are doing it because they very much want their grandchildren to grown up Catholics and they hope that their son will return to the fold.
Now they have also accepted that your reasons are totally hypocritical, so I don't see them as white than white either, but I really think that it sounds as if you are pissing them around a bit.
Your mum not being there is understandable in the circumstances, but is anyone coming from your side of the family/your friends? If not I can totally see your MIL's upset. If your frind who you have asked to be a godparent can't be bothered to come then they won't be acting as a godparent in any case. A Godparent is not your best buddy, but a) needs to be a Catholic, and b) needs to make some fairly hefty vows on your children's behalf. I think you should choose someone else.
(NB I think what you are doing is totally crass. But then I am an ex-Catholic atheist with very religious parents who would love for my children to be Christened, and brought up in pretty much any faith).