jabber- I'm not sure that I understand.
If you go along to your DSS first day, there will be, at best, tension between the adults, or at worst, an almighty scene, either of which will affect your DSS, your DD and possibly other children as well.
There are three adults involved, any of whom can make a choice that would avoid that happening. Your DPs ex could choose to accept your presence, your DP could ask you not to go along or you could choose to step back and stay away. You and your DP have decided not to make a decision that will prevent upset for the DCs but you expect your DPs ex to do so.
And yet, you consider yourself a 'second mum' to DSS, say that you have raised him, are committed to him and love him like your own?
I don't understand why you wouldn't do everything you can to avoid causing him distress/anxiety/upset if that is the case?
It's not a state of mind I can relate to at all - I certainly don't have the same level of emotional attachment to my DSS, and yet my primary consideration when making any decision is how he will be affected by my decision. I can't control or influence how his mum behaves, but if I can make choices that avoid negative experiences for DSS, then I would always do that.