My personal guess is that the RE4 remake won't be fully intact but it'll have way more kept in than RE3's remake and probably a bit more than the RE2 remake.
Capcom knows people were pissed about the RE3 remake's brevity and unlike RE2, where the original game hadn't been easily available on current systems AND you had the mystique of it being a long-awaited remake finally coming to life, RE4 has been easily available on current-gen and next-gen platforms consistently since the PS2 port, it's Capcom's equivalent of Skyrim or GTA V. So they are going to be a bit more careful.
This is all just conjecture on my part, so take it with a grain of salt but here's what I personally think's gonna be cut...
1. Assignment Ada. Despite it being one of the OG minigames for RE4 since the Gamecube, it's not tied into the main storyline like Separate Ways is, nor is it as iconic as The Mercenaries. While RE3 remake got Mercenaries cut despite being where the Mercenaries minigame started, that game was a clusterfuck and I still suspect it was originally going to be a story DLC for RE2 that was supposed to be just the SparkNotes version of the original RE3 before they spun it off into its own game.
2. The fork in the road after the iconic farmhouse stand-off, though I think they'll cut out the El Gigante part and keep the Bella Sisters since it's slightly longer and we already have another El Gigante fight prior to this.
3. The majority of the QTE's will be cut, save for the really big ones that still hold up like the first Krauser fight. I'm 50/50 on whether or not they will cut out the laser room since it was well-known but it was also one of the goofier elements of RE4, and I think this remake is trying to go for a more somber grimdark feel overall.
4. Relating to the previous point, the infamous Salazar Statue chase near the end of the Castle will likely be cut and is the most likely of all the QTE's to get removed.
5. The extra costumes will be different. I might see them let Leon keep his RE2 outfit and the Mafioso outfit but I don't think we'll see Ashley in knight's armor or dressed like a popstar.
6. The village and the castle will have the most intact in the story mode and the Island will probably have the most alterations since it's often seen as the weakest part. The really big moments on the island like Oven Man, the Regenerators, the boss fights, and maybe Mike's helicopter raid will be kept around in some form but the rest is up for grabs, IMHO.
It's just not realistic to think it'll be a 1:1 remake. They've shown twice they're not interested in giving games move dev time to make it complete. Although there's no indication 3make was going to be anything more than what we were given. It's better to expect it at this point and be surprised. I'll buy it and give it a chance. I know it'll be better than 3make, but even from what they've shown I'm not crazy about it and with how many times the game has been release and you can download the HD project for free, it feels very redundant.
The Island is the weakest part of the game since it's just not interesting, but it's not from lack of content. It has no atmosphere or character like the village and castle. I've always felt the castle should've been the conclusion. Although, the mines are very important to the plot even though I don't enjoy that area.
As for your points. If Assignment Ada is in it, it'll be DLC. I wouldn't doubt it's not even in it at all honestly. Both RE2-3 didn't get post game content worth even messing with.
The fork I feel like they'll cut one side of that and make it more linear. Which way that goes is anyone's guess. Personally I always do both whenever I play it.
I feel like the laser hall will make a return since it's become such a meme. Although I definitely see them trimming down or removing the QTE fight with Krauser.
The statue will get removed. And honestly it's probably the biggest cringe nonsense in the original since it's really out there and cheesy.
I think his RE2R outfit will be in there. It's using the same engine so I wouldn't doubt they just repurpose the existing skin and upscale it a bit.
In a perfect world, the Island will have all the excess fat cut from it and give it better pacing. That was always my issue with it. It's not a slog but there's areas that I just don't like. Like the first half of getting with Ashley up to the wrecking ball area I hate.
Everyone has their fears of cut areas, but I'm more concerned with cut enemy types which seems like it would be the first content that would be on the chopping block. Will there be three types of head parasites like the original? I don't think it would be game breaking stuff but stuff that'll irritate you in repeated playthroughs.
Mercs has to be in it since it's such an integral part of the RE4 love people have for it and since coomer DC Douglas leaked the Wesker artwork.