The Substitute Princess Becomes the Empire’s Most Beloved

Maybe he's on high alert because of the last assassination attempt? Moreover they don't really interact much and I guess he's respecting her decision?

This. Not like FL was any healthier in the first timeline and they actually had the wedding night. That should have been enough to notice.

It's obvious Marguerite. She's OBSESSED with wanting to REPLACE FMC and when you look at it, they share a hair color, she has shown herself to be a sociopath not above COPYING FMC if need be (though still ANGERED that the one thing she CAN'T copy of FMC is her BLESSING......and of course it's IMPERFECT as her disguise falls apart as you said by her facial features, that SADISTIC smile and face she always makes "gives her away HEAVILY" because it's her NATURE, it's easy to see how that is one thing she'll NEVER overcome! Her true nature!

Sadly FMC HAS to endure it BECAUSE FMC wants to know "who helped her!" (Remember the bitch's OBSERVER back in the other kingdom said "someone helped her sneak out the castle and get through the border to the Imperial Castle with such ease!) Don't forget FMC DIED by her hand BECAUSE she was able to sneak in so easily "with so many HELPERS" she already ID's Edda obviously as the one who let her in, but the bitch had "free-leeway" to waltz into the Imperial Royal Castle WITH Assassins and NO ONE stopped her as she easily endangered the Royal Heir and MURDERED the Empress! The only way you can GUESS she survived the attempt was using the Child as a Bargaining Chip because you KNOW the ML would kill her and EVERYONE who helped her infiltrate and murder the FMC like that! Even SHE knows this WILL result in her death, hence her CONFIDENCE that she WOULD get away with FMC murder and get everything she wanted! Note the guy she was talking with, her co-conspirator! Is likely the Reason all of that even SUCCEED (even IF Edda)

As annoying as it is, sometimes this type of shit happens! FMC HAS to be submissive BECAUSE she knows her sister is a Sadistic Bitch who "will know something is up if she doesn't FEED her ego." The sister's GREATEST weakness is her Cruelty! As in she does ALOT of needless things for the sake of it and to get off on it! She didn't HAVE to GLOAT to FMC when she murdered her, but she did, she didn't HAVE to do alot of things, but she did! Hell this INFLITRATION resulting in her not being PUNISHED as you said is one big giant "counting on her ALLY and FMC acting in the manner she HOPES she will continue to act, remember she's been using Edda and is 100% unaware Edda has TURNED on her! It's like what happened between Mai and Azula in The Last Airbender! Azula is too use to people FEARING her that she can't even FATHOM she herself can be played! Comes with ARROGANTLY winning TOO MUCH! It creates a BAD sense of Invincibility! Plus she's in a VERY dangerous position as you pointed out, she is a Princess to a VESSAL KINGDOM! She may THINK she has her "ally" outwitted, but he's in a PERFECT position to throw her under the bus and she has YET to full understand her father WILL throw her under the bus if the Kingdom and more importantly to him HIS HIDE are on the line! His fault for INDULGING her sociopathic tendencies and sadism and then FOOLISH believing she'd "take it lying down and be COWED by him EASILY if he demands it! The largest reason WHY i jut her BRAZEN she was to be involved in CONTINUING her bad behavior with FMC despite everything! Lets not forget, the moment FMC became Empress, her little GAMES with her was finished, Continuing them would result in a VERY BAD ENDING for her because FMC is HIGHER in the food chain then her now! But you KNOW Sadistic Bullies NEVER accept these outcomes! And the only reason she hasn't SUSPECTED the jig is up and she's been ENTRAPPED is BECAUSE FMC "bit her tongue and played HELPLESS!" Even if ML has to "save her" the fact she shows "Weakness" makes the bitch lower her guard because "just like old times" kicks in and it makes her do and think VERY STUPIDLY! She's still on guard, but not as onguard that she WOULD have been if not for all these tricks! FMC admitted a part of her "mercy" to Edda was less her FELLOW VICTIM status to her sister and more because she "KNOWS" her sister well. She KNOWS she has a bad habit of indulging in STUPID EVIL if she thinks she can get away with it. And after her horrible behavior to Edda for all those years that she HAD HER HUNTED DOWN WHEN HER PARENTS FAKED HER DEATH TO SAVE HER FROM THE TORMENT JUST TO CONTINUE TO TORMENT HER, SAME WITH FMC! It shows she hasn't CHANGED, and can be prone to stupid mistakes if she lowers her guard and she WILL lower her guard if FMC allows it! She still HAS to pretend she has a "LIMB" around the bitch BECAUSE she knows it brings her JOY that "she arranged that DESPITE her being EMPRESS!"

But again, She is a SMALL FRY compared to who is helping her, likely someone wanting to DETHRONE the ML and sees her as a valuable pawn for that! Likely assuring to keep NOT TO MUCH HANDS ON because he knows SHE is needlessly stupid Evil and it would risk his involvement being found out. Plus FMC is playing a DANGEROUS game as ONE mishap she's unaware of, and the bitch might play in a different tune she didn't expect! She NEEDS to conclude the investigation into her "co-conspirators" QUICKLY before she gets SPOOKED (And before ML finally has ENOUGH and goes "damn the consequences" to AVENGE what the bitch put his wife through and CONTINUES to try and put her through! He can only be PATIENT in keeping his SWORD ARM under control around the bitch!)