I basically agree with all your comments, except the notion of how to know when the Passover is. It is the 'sun' that marks the date, 'spring equinox', year in year out, the time to count when the Passover feast takes place. 15 days after the spring equinox begins the Passover. The moon is fickle and not reliable for calculating events that are fixed year in and year out.
The practice of the goddess Ishtar and fertility rites of quick like a bunny and eggs celebrated at the Spring equinox did not begin with the death of Christ. Some people took others religious practices and folded them what they claim to be Christian.... Well there is not one hint or shred of historical evidence that these practices were ever intended to be practiced by the followers of Christ.
I once asked a modern day prophet what Christians were doing celebrating 'Easter', when in fact the word was never Written, but the translators got fast and loose with the word Passover. He told me it was for 'the children', they like to hunt Easter eggs.
Example....Exod 12:18 In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening.
Groups like the Kairites keep track of the correct days for the "official" calendar celebrations. shabbot shalom