Also, Spock never went from Vulcan to Earth - he went from Vulcan to a point where the Enterprise was left after her adventure inside the wormhole. Where that point might lie, we don't know - wormholes are like that. In the Charts setup, though, the ship would be going towards Vulcan when going towards V'Ger, making Spock's task easier.
(Also, we don't really know when Spock launched. It happened after the Klingon fight, but before the clock started ticking for Starfleet; might have been days or even weeks before the main body of the movie! Although why Spock would have headed for Earth, rather than for V'Ger directly, we can't fathom. If he intended to get a ride from his old pal Kirk, why didn't he tell him that? So we get even farther from the idea that Spock would have been in the process of reaching Earth from Vulcan in unduly short time.)
As for the Klingons, we have no idea whether the hopeless fight of the trio was the first for the Empire, or the last in a series. Parties engaging V'Ger would be hard pressed to file a report (one wonders whether the one from Amar ever reached its intended recipients), and Klingons wouldn't be depending on the experiences of their peers and competitors anyway. But the Charts try to make that easy, too: the movie confirms that V'Ger came from Klingon space, but here the route at least goes through that narrow extension with which Klingons arm-wrestle with Romulans, rather than through the bulk of their Empire. (Okay, it was supposed not to go through the bulk, but Mandel decided otherwise.)
The "regular occurrence" thing I don't see at all. The anal-retentive Sheliak cover contingencies in their legal jargon, not regularities...
So Tau Cygna V was founded by the Artemis; that's closer to being proof that travel in the direction was rare than anything else, as we're again dealing with a group of isolationists. And when the Artemis set sail, long colonization sorties utilizing cryosleep in combination with warp were still being conducted, as per that story about Harry Kim's ancestor; covering of hundreds or thousands of lightyears, possibly much more than exploration ships, would appear likely.
Timo Saloniemi