The list https://itch.io/games/newest is a different sorting of the list https://itch.io/games and this is literally a list of all indexed games. Saying it is on the list and saying it is indexed are equivalent.
You will find projects that are indexed and have no files. It is not a strict process. Technically you violate https://itch.io/docs/creators/quality-guidelines , so anything can happen. Itch will not tell you. They will also not tell you, if you are still on the waiting pile or if your project was processed and deemed to be non index-worthy.
Only indexed games can be searched. Search is by literal title. Search with exact title or partial title of SHELLSTORM: THE GREAT WAR will find your game. Though, searching with "war" probably would not find it. Results are capped at about 60 results and that's a common word. Searching your developer name Hypermad would also find the game.
You can filter down the https://itch.io/games list by selecting tags and other things. That's not a search. That is just displaying the indexed game list with filters. It is still sorted by popularity.
Devlogs appear in the https://itch.io/feed and in https://itch.io/devlogs . I do not know if all will appear in the devlogs section or the feed. But they will appear to your followers in feed. I assume that devlogs attached to an unindexed game will not appear in the devlog section, nor the global feed - it would defeat the purpose of not being indexed. But, this type of promotion is moot. Your own website will have more traffic than an unattached blog posting (those exist too. Please read other threads how those work and better yet, just browse that section and see for yourself what is there). Promotion within Itch is very limited and mostly non effective. You can make a post here https://itch.io/board/10022/release-announcements if you like. But look at how many people read such posts.
I linked you two relevant sections of the quality guidelines for why offering a wallpaper in the game section is against guidelines. Just read the headlines of the paragraphs. "Avoid only uploading keys or links to other stores" "Do not use unrelated tags or classifications to promote your game". You did not even upload a game! Also, the very first paragraph of the guidelines: "Avoid publishing your page before it’s ready".
You mentioned that offering a poster might break Itch's rules.
Offering a poster is not against rules. Calling that poster a game is. As the mod told you. "I wouldn’t recommend having a file that’s not the game, as the algorithm might think your game is finished an available to users, even though it’s not, which might be poor user experience."
Having nothing to show means, that you have no game to show. Itch is a download store. There is no game to download. Nothing to play. You are merely creating a game page without a game, that would link to your homepage. This is not the intended usage of Itch. And if they enforce their guidelines, that page would not be eglible for indexing. They probalby would not enforce it, since your release is in the future and you just did not use the tools to have a future release date.
Itch has a negative stance against "optimising". They want things to be "natural". You can read that undertone in some threads here. So just publish your demo and tell people, if and when your game will be available here on Itch, and if they will get a Steam key with their purchase here. Uploading "in development" versions of games is the usual way. And for those, the development logs make sense.
And my initial post was to warn you against orchestrating a minutely planned release. It might not work, since you can't guarantee being indexed at that date. You also might get temporarily de-indexed by updating. But then again, being indexed will not matter for your promotion, since as I said, people will not type the title of your game into the search box. They will follow the direct link you promoted. But if all that is here, is a demo, why should they even visit Itch for your game, instead of Steam. Maybe I misunderstood what you plan to do.