Hey supreme. I think I can help out too.
You're list is really comprehensive, but there are even more versions/credits that need preserving, with most being international versions. I'll list some more.
The Last War-An international version has been said to exist, that is much different from the U.S. version. Both this, and the U.S. Brenco version need preserving.
Gorath-The Brenco version is in need of saving, as does the international version. There is an international trailer for the film on Toho's R2, but it is unknown whether or not this version has ever seen release anywhere.
The Human Vapor-Brenco's version is in dire need of a preservation.
Atragon-Toho's international version under the "ATOROGON" title (which oddly enough, Toho re-used in Godzilla:Unleashed for the Nintendo Wii) has never been released in the states. Both that, and the AIP version under the ATRAGON title, need preserving.
Matango-Same case as is with ATOROGON and ATRAGON.
Dogora-Same case as the above two. It is unknown if an international version even exists, or if it has ever been released.
Ebirah, Horror of the Deep-While the Sony DVD does technically use this version, the original Ebirah title sequence was replaced with a generic looking "Godzilla vs.The Sea Monster one. The credits were also re-done, though they are very faithful to the original international credits. The "End" card is also completely different on the Sony DVD. The Walter Reade release would probably have to be reconstructed.
Son of Godzilla-Same case with Ebirah, though the title sequence is extremely faithful to the international title sequence, and is only slightly smaller. As is the case with the Walter Reade version of Sea Monster, a reconstruction would likely have to be made.
Destroy All Monsters-Since MB didn't do the job themselves, both the AIP and international versions still need restorations. The AIP version would be impossible to restore without 16/35mm elements in decent shape (since the AIP credits seen on the MB release are in terrible shape), but the Blu-ray release provides all that's need to restore the international vesrion. The international credits are in HD, and look damn good, actually.
Space Amoeba-International version is MIA. It is unknown if it has ever seen release. The AIP version under the "Yog" title (as you previously mentioned) is also in need of saving. Like many of these original U.S. versions, it only ever had a pan and scan VHS release, which makes a preservation/reconstruction, tricky.
Godzilla vs. Hedorah- Minus the pesky copyright info, and omitting the on screen text for Ken's haiku, Sony's DVD did a fine job of recreating the international version of HEDORAH, for the most part. All that really needs saving now is the AIP SMOG MONSTER version. The best source outside of 16/35mm is the LaserDisc release.
Godzilla vs. Gigan-Regarding the international version, it's really complicated (Tamura or I will update the English Versions Guide soon enough). The Cinema Shares release of GODZILLA ON MONSTER ISLAND is definitely in need of sainvg/preservation. This version has never seen release on VHS in the U.S. Most fan-circulated copies are taped off of TV, and generally look awful.
Godzilla vs. Megalon-This is a tricky one..some claim (same as with all of the Cinema Shares releases) that MEGALON was first released uncut theatrically, and then later, was cut for re-releases, as well as for TV airings. It is possible, that using Toho's uncut international version, and an HD version of the CS logo, that an approximation of the original CS release could be made. As for the international version, it has never been released on home video, though a restoration may be possible if we ever get that special edition release of MEGALON ,which was to include the CS logo and credits, as well as the international credits all in HD. If the BD release of MEGALON never happens, it won't be possible to restore either version without 35mm elements.
Godzilla vs. Mechagodzilla-Sony's release of the international version is very well done, like HEDORAH. The original Cinema Shares version of COSMIC MONSTER is definitely in need of saving, as is the even more rare BIONIC MONSTER version (which may not even exist anymore, if there are any surviving prints, I don't think it's something anyone in the fandom knows about). 35mm elements would have to be used, since COSMIC MONSTER has never been released on home video outside of faded, pan and scan releases.
Prophecies of Nostradamus- The U.S. "Last Days" cut deserves to be preserved, at the very least (as awful as it may be) utilizing the lone LaserDisc release. The international version would have to be recreated, using a high quality transfer of the film, and that will most likely never happen.
Terror of Mechagodzilla-Tamura has restored the credits of this version, so all that would need to be done is add these credits to a nice looking transfer of the film. A U.S. release is highly unlikely until somebody else gets the rights to the films that Classic Media has, and even then, it's a stretch.
Return of Godzilla-Preserving this version is very difficult, since it has only been released on PAL VHS (In both England and Finland). It will be impossible to restore without 35mm elements, or a really high quality BD release. The U.S. version (GODZILLA 9185 could probably be recreated using a version of the Monsters HD airing in 1080p, but even then, it would require more work to make it match the theatrical/home video versions.
Godzilla vs. Biollante-the HBO releases contain the uncut international versions..but because it was never released on anything after the LaserDisc format, a restoration is almost impossible until an HD version is available, or if 35mm elements could be utilized.
Rebirth of Mothra III-This was available for playback in HD on crackle.com, for a long time, but has since disappeared. It may never seen a home video release, so the best option would be for someone to somehow record the online streaming version (if that's even possible) in an effort to preserve the international version.
A few of these titles will be impossible to restore, without 16/35mm elements. Those included are The three Brenco titles (The Last War, Gorath, and The Human Vapor), The two Walter Reade releases of the Island Godzilla films, and the AIP releases of "Yog Monster From Space", "Attack of the Mushroom People", Atragon, "Dagora the Space Monster", and Destroy All Monsters. Except for Atragon, all of them have at least gotten VHS releases, so they can at least be preserved via DVD-R copies.