I'm sorry, but your first claim, that Christmas is a co-opting of a pagan holiday (Sol Invictus) is just plain wrong. It predates Sol Invictus. Emperor Aurelian established Sol Invictus as a holiday in 274 AD.

Hippolytus of Rome (d. 235 AD) claimed Jesus was born 8 days before the Kalends of January, which corresponds to Dec. 25. It is vastly more likely, and much more widely accepted at this point, that Dec. 25 was chosen because Africanus (author of Chronographiae, an early attempt at a Christian timeline) and other early Christians believed the Annunciation was March 25. They just added 9 months to that and bam, December 25.

If anything was intentional about the 25th in particular, it would've been due to contemporary Jewish beliefs that Prohpets died on the same day they are born or conceived. Believing that Jesus was conceived on the 25th of March, the parallel 25th of December would not only have been chronologically accurate, but spiritually significant.

These early Christians existed well before the establishment of Christianity as the Roman state religion. There was a substantial desire to distance themselves from Pagan practice at the time. Virtually all sources that it relates to Saturnalia and Sol Invictus, outside of a single margin note in the 12th century, are post-enlightenment.

Edit to address Easter eggs, in particular: Undeniably the symbology of the egg as representing life and death predates Christianity. Frankly, it predates the Roman religion too. It's more likely that eggs came from Persian cultural practices, spread to middle eastern churches, then gradually migrated west.

That's just how culture works and I honestly don't see what the point of bringing it up is. Only the most simple-minded evangelicals would be scared of what amounts to adiaphora.

Are we obliged to belive that every religious practice from every religion ought to have been instituted specifically by its God? Religion is for us, even most religious people will tell you that. Islamic prayer forms are derived from Coptic Christians, Jewish and Christian thought intermingled; Hindu, Sikh, and Buddhist thought has at various points cross-pollinated in fascinating ways.

I truly believe more non-religious people should read David Bentley Hart's Experience of God. I think, if you can put up with his snark and dense prose, it would help a lot to understand what it actually means to believe in a God, rather than bottom dollar examples like rural evangelicals and Islamic extremists.

As far as Christmas trees go, the fact that we do not see a single example of one until the 15th century, leaves me confident that their adoption wasn't related to co-opting ppaganism. European Paganism was dead and buried by then. Any practices that remained would've been perceived as cultural. The Christmas Tree probably had more to do with wealthy Protestants trying to distinguish themselves from Catholic Christmas traditions more than anything else.

Heck, the Vatican refused to put up a tree until the 1980s. That rivalry runs deep.