If you really want to, I'm sure these kind of similarities can be found. But that's how symbolism works. We take an object, image, etc. that has a certain meaning, and reuse that object, image, etc. in a different context to transfer that meaning to the new context.

For example, Tea Party protesters might stage a protest against the government on the 4th of July. This has two clear historical allusions. Now, in Dan Brown's logic, we might use this to conclude that in fact the modern day Tea Party is a continuation of a centuries old organization commited to fighting British Monarchs ever since the Barons forced King John to sign Magna Carta, or we might conclude that the modern day Tea Party uses historic imagery to associate their movement with older, but unrelated movements.

So, people living in the Roman Empire will have had similar ideas about religious imagery, about spiritual narratives, etc. This all was related to already established forms of religiosity, and this had an impact that reached several new religions established around this time.

Dan Brown is smart enough to pick up on some of them, but has a nasty habit of concluding very strange things based on these simple observations.

The only one I am willing to directly speak on is number 3. Sunday is not explicitly to honor Ra.

The Egyptian astrological model uses 7 planets, each of which took turns ruling different parts of the day, however the planet that 'went first' got to name the day. So on the day that the Sun ruled first, they called it Sunday.

This naming convention lived on through the Romans and later Europeans however they adopted it to their own languages. Sunday later became "dies solis", and then through a mess of language transmutation that I am not qualified to explain, later became our modern Sunday.