Why did Gene Roddenberry think it necessary to bring in two characters who were totally new to the series, dedicate so much of the story to and at the same time get rid of them like they were just a one episode character?
In the script of the unfilmed telemovie, "In Thy Image", which morphed into ST:TMP, the Ilia Probe
doesn't get whisked off with V'ger (or N'sa, as in NASA). It reverts to a small burnt out mechanism, which had originally been the floating core of the hot/glowing, tentacled probe of light that had snatched the real Ilia from the bridge. Chekov had named that first probe "Tasha", because it had a pearlescent glow that reminded him of his Aunt Tasha's pearl ring he knew as a child.
As they investigate the burnt out device, suddenly the real Ilia materialises, unharmed, just as Decker and V'ger merge.
Will Decker does vanish with V'ger to explore the unknown, as in TMP, but as his character was a featured identity in all twelve completed scripts and proposed treatments for the "Phase II" series, just how he was planned to return had yet to be decided. Whichever script would had been chosen to follow the telemovie would have had an added scene, wherein Decker was somehow retrieved, to be reunited with his ship and Ilia.
As it was assumed that William Shatner was unlikely to remain with "Phase II" as a regular, beyond the first half-season, plans were on hand for Decker to become captain and Kirk to be promoted as admiral (to make guest appearances). That promotion idea ended up being used for TMP, too. Shatner had made it known, perhaps for salary-raise leverage, that he was more interested in pursuing a movie career and thus was unlikely to want a long stint on TV with "Phase II". (Unless it was wildly successful, of course.)
To preserve the Vulcan, Xon, for the TV series (or future movie sequels), they invented the disposable Sonak - and asked the actor already cast as Xon to guest as Commander Branch instead.
Oh, and of course the super advanced aliens fuck it up and send it off to destroy the Earth.
You've totally missed the point. The living-machines' planet just repaired V'ger and it returned to its mission to learn all that is learnable. But its amassed knowledge enabled it to achieve consciousness and sentience - and began to seek out its creator. NASA was from Earth, but V'ger saw the human populace as an annoying infestation getting in the way of it being about to touch the creator. There was no mission to "destroy Earth", but V'ger mistakenly didn't recognize the "carbon units" as sentient.