I don't think you will find that a CME would cause a substantial change in orbit of a comet - not in terms of any presently known orbit object being deflected enough to hit the earth at any rate. Comets quite often break up when nearing the sun without any association with solar flares - it's normal.
Occurances of two comets being visible at once are not all that rare. I've seen double sitings myself and I really don't know what you think a double siting implies unless you believe in omens! Visible comets do not cause magical effects and disruptions on earth any more than those which are below visible threshold; they just give us an opportunity to wonder and gaze at these graceful ladies gliding through the heavens at cosmically safe distances.
There are thousands of comets going around the sun in elliptical orbits ranging with periods of as little as a few score years to a few tens of thousands of years. The longer orbit ones are of course the ones that generally get announced as newly discovered and as it is often difficult to work out the exact path of a comet until one is almost upon us, this accounts for the short notice - as you say sometimes six weeks. One must remember that a tail on a comet is not visible until it gets relatively near to sun when it radiations causing the ensuing comet debris to glow.
I will certainly agree that the impact of a comet or asteroid may have contributed to sudden changes in past civilizations and I am mindful of an asteroid that landed near Rome a few hundred years ago, and of the accounts by the Mayans of a blinding object hitting the earth near the Atantic ridge around 8498 BC as described in Otto Muck's book "The Secret of Atlantis".
So I would say sleep well at night and be careful of scaremongery stories you read on the Internet.