I remain dubious that they would have him "join" with V'ger and then come back. What's the earliest script date you've seen where he does that?
My "In Thy Image" script, as sold by Lincoln Enterprises, is in storage, but it was for the telemovie. Decker definitely departs with V'ger in that. The other script I have on hand is the Shooting Draft of ST:TMP, but only the replaced, coloured, "revised" pages are individually dated. It has a secrecy memo from August 4, 1978, and title page from July 19, 1978. It contains many multi-coloured pages of replaced script revisions, the last new (blue) page is for the bridge scene, of Ilia being consumed by the Probe named Tasha (August 28, 1978). Earlier pages have later dates. One set of Appendix pages have all of Sulu's lines marked up in blue pencil, so were once owned by George originally(?). The final third is essentially the same story as the all-white pages which concluded "In Thy Image". (I have not yet compared the two scripts side-by-side.) All thirteen "Phase II" episode scripts and synopses feature Will Decker and Ilia. So he definitely comes back, or all those scripts would require a replacement first officer. In interviews, Roddenberry explained that they didn't yet know how Decker would return, only that we are told he was merged with V'ger. In this script, the fully-restored Ilia says, "[to Kirk]: I felt Decker pass into V'ger. Together, they saw to our safety." Memory Alpha describes the scene differently. I am thinking this is from Alan Dean Foster's synopsis: "Kirk, Xon, and Decker are beamed back onto the Enterprise, as is the real Ilia. The robotic Tasha appears as well, but inert and dead. Kirk implies that her returning to the ship, instead of to Ve-jur, is what she wanted." It is quite reminiscent of what TNG's writers did with Picard at the end of "The Best of Both Worlds, Part 1". They deliberately painted themselves into a corner, then went on hiatus. Roddenberry was not yet concerned too much with how to return the character of Decker, and perhaps the only inkling of why Decker is able to survive the experience is his affinity with the New Human Movement hive mind concept, briefly touched upon in the novelisation of TMP.
As I've said elsewhere, given the gray-area when the Star Trek II show was shifted into TMP that there might be drafts of the script that fall into the former's timeframe would not be surprising even if they were nominally feature scripts. As was documented in the Phase II book, Paramount let the TV show coast for a while even though the writing was on the wall because they hadn't announced the end to their 4th network ambitions and were shooting widescreen tests for a movie even as the TV scripts were still being worked on.
Re above. I checked my own archives and the earliest so-called Livingston movie script I have is listed as "undated", and has the nude Kirk & Alexandia swimming scene at the top and ends with Decker merging with V'ger and Ilia being returned. The way they plan to "join" V'ger it is beam the "entire molecular configuration" of someone's brain into V'ger, losing everything else in the process. It's out-of-nowhere technobabble, but the hint that Decker could be restored comes here: The above is similar to a supposed Oct. 20, 1977 draft (poorly formatted) I am linking here, but which has the original Robot's Return ending where V'ger just goes, "humans, meh" and leaves. So we can assume the script I have is post that date, but where it lands in the script chronology is not clear. But at this point they're clearly thinking feature because there are no act breaks. So, this confirms what Therin recalled.
Explored time, space and other realms of existence, eventually becoming the "creator" of its own universe and life.
TMP Decker? Or Phase II Decker? Only asking because they had a Phase II story featuring Xon in their last run.
Thanks! Although I would've preferred a Phase II story over one on the Decker-Illia-V'Ger lifeform. I prefer questions than answers. I like that it's left up to the audience to wonder what happened to them. Also I want to see more stories set in the Phase II universe... the Trek that never was!