Let me answer the 50/50 thing. I grew up in a 50/50 situation. Although my step-mother was not very kind to me and my brother until she had her own children (my step-brothers), 50/50 worked very well, especially as my parents were very good and civil to each other - just as you and your WH are with regards to S! In my opinion, you are already modelling the BEST 'divorced parents' behaviour possible. BRAVO to you, because I swear, very few parents achieve this.
Also, you will have to adjust the 50/50 thing over time. Perhaps when schooling starts, it will make sense for that to be 70/30, in your favour.
As an example, my brother and I for most of our schooling years spent Wed and Thurs nights at my Dads. This way least interferred with social and sporting activities on the weekend.
My parents also decided to live one suburb apart, so that transporting us was a quick 5mins, and we could do it easily by bus as soon as we were old enough.
I always saw my two houses, my two bedrooms, two birthdays!!, as a source of richness. Kids don't suffer from having two of everything - well, I didn't!
Today we are one big happy family and share all special occassions together and live, literally, as an extended family. That level of closeness and intergratedness took 2 decades to achieve, so it IS possible, it just takes time...
