Regarding lying to your S... I don't think I misunderstood...
Was it SD that said, how did S take the news that you lied about Santa, the Easter Bunny, and so on?
That seems a little ridiculous to compare the two, but if we're strictly speaking about lying... knowing that Santa is not real can be quite traumatic to a child... why do we even encourage these types of things? But then... a child grows up where EVERY kid believes in Santa...
It is what it is...
If your H beat you, would you tell your S why you and H are separating? Even if S saw the bruises?
If your H was going to jail for murder, would you tell your S why H was going to jail?
A kid who lives in a war torn country and gets to witness freshly decaying bodies or other traumas...
If you are passing an accident and you know one of the occupants died, do you tell your S?
As hard as it is, we cannot raise our children in bubbles... unless we raise them in bubbles...
I know that there are many different viewpoints on this and I absolutely understand your desire to remain truthful to your S.
Child Therapists / Psychologists who specifically deal with D tend to suggest that the WHY the parents are divorcing is less important than the children knowing that the parents still both love the children and will work together to raise the kids.
That's from testimonials directly from children and adults who are survivors of D parents.
Yet... I can really say that I do not feel traumatised by having found out my dad had an A when I was at a young age. I'm probably more traumatised by my mother's alcoholism. I don't know why, because I know my mom daemonized my dad for the A. Maybe that's why. Because my mom daemonized my dad, and I wanted to love him, just as much as I loved my dad, and so I blamed her for being the one to let me know...
Telling your S might be just as traumatic to him than him finding out from another source as opposed to finding out sometime in the future that you lied... and being at an age where he can understand why you would want to protect him from that info...