I think Engagement Ring appeared in the menu after Carrie received bad ending, like some sort of bonus item for the new game. I don't think it could trigger bad ending, more like to help new player, who previously got bad ending, achieve good one on the next playthrough. Actually this brings an interesting topic - could it be, that all playable characters had bonus items, they received after you completed game with ceratin ending...? Also, Engagement Ring clearly confirms, that Carrie's bad ending in the early version of the game was the same as in final version. Through this, it is possible to assume, that all endings in CV64 were taken from the old storyline.
I think Contract didn't triggered anything in terms of endings. Early previews hints, that Renon wroked pretty much the same as in the final version. You summon him and give him "something" in exchange for items. Now, "something" is most likely your soul. In old system it is described, that you gave him points for items. I think points somehow meant pieces of your soul (I know that ridicolous, but it's the best explanation possible with information on hand). Giving him more than 30 pieces of your soul, triggered battle with him in the final part of the game. However, it makes Gold collecting rather obsolete. At least from the old preview, it seems, that you exchanged "something" for items and Gold wasn't involved. Or maybe you exchanged items for "pieces of soul" and, also, should have enough Gold, but it's redundant. Either bartering items for soul or buying them, makes much more sense, then both of those things merged. Of course, it's possible, that developers haven't figured it out themselves and "points" system was a template. Besides, I don't see why it should have been cut, since it doesn't make mych difference from the buying items for the Gold, just different statistic involved. Most likely, they decided, that Gold was useless with the "points system" and replaced it with buying items. Also, I think, they also figured out, that having constant access to the merchant, kind of throws the game out of balance. Or maybe it was because initially the game should have been longer and thus constant access to the healing items was justified...
As for new Towers from LOD, they designs are clearly meant for LOD only, but maybe they should have been present in the original game as well, but with radically different designs. There is no way to be absolutely sure about it, sadly.
If you talk about double doors opened with Science Key 3, then there is some items, I think. The same thing exists in Tower of Execution.
I attempt to find something about CV64 for DD on Japanese sites, but so far no success. Japanese Wikipedia doesn't mention CV64 being developed for DD, as well as other sites I visited. Granted I haven't visited very much and I plan to continue search, but I doubt, that CV64 was actually in development for DD. Maybe only in conceptual stages. But the actual development was for the cart only. I think, that "Moon phases" was more or less something, that was only in imagination of developers. It would had make the game too complicated.
As for dead villagers - there is a thought: in early preview, producer of the game mentioned, that in CV64 you will be able to find items only in walls and bodies of the previous adventurers. Could it be, that villagers corpses, that you see in the game, could have been looted for items, like subweapons? It's fairly logical assumption and give corpses some meaning in the actual gameplay.
I think, that CV64 was meant to be the main Castlevania games, but rushed development and overwhelming success of the SOTN kind of spoiled all plans for the future of Konami and, somewhat, led to the cancelation of the Ressurection and rise of the IGA.
I too think, that stake subweapon, should have been used to finish off vampires.
I don't see nothing wrong with talking about this - such discussions give rise to the interesting ideas and allow to understand better, what developers had in their minds back then.
COTM review so far is not finished and I will serach for more information on Japanese sites. So, far I found one interesting Castlkevania fan-site called "We Love KCEK!!". It is fan-sire for the series as a whole, but have special section for KCEK developed entries. There are hilarious gallery with screenshots from CV64 / LOD, list of rumors about the game (confirming female design for Coller), interesting measurements of characters height and trivia section about some misteries in the game. For example, mirrors in the Villa shows not reflection, but a copy of the room, through dark filter. And this site has cute chibi-Carrie jumping from Water Dragon on the main page.
Pictures were taken from gallery on that fan-site.