My suggestions, coming from the other side of the fence where I (and some of the other players) feel that the DM plays a little too fast and loose with the rules, and makes changes to things that we think ought to be "canon" for the well-known world we are playing in:
1) Be willing to consider that the player may be right. Allow him to make a brief argument referencing the rules. Then make a ruling. Make a mental note of how often you rule against the player versus how often you change your mind and agree with him, and try (later, outside the session) to assess whether you're being particularly harsh and/or truly weakening one character's abilities relative to the others'.
2) Be firm if you still disagree with him. If he still disagrees with your ruling, tell him, "I need to ask you to go with the DM ruling for the moment and we can discuss it more later outside of game time, to figure out how we'll play this type of situation in the future."
2a) Try to offer the player another way to reach his objective. Say something like, "Look, the rules say that you give away your position if you attack from hiding. If you then, in full view of the enemy, duck behind the same tree, they are going to know where you are, even if you are so well hidden that they can't perceive you. Thus you do not get the advantages of being hidden in that case. Now if on your next turn you stealthily move to the next tree and hide there without being noticed, and then attack, that would be unexpected and give advantage."
3) Ask players not to use the Monster Manual at the table, and to avoid using metagame knowledge about monsters. That said, try not to mess with well-known monsters in a canonical setting without a really good story justification. If you're playing in a canonical setting, Mummies are going to be something that most adventurers will know the legends of, and the way that Mummies are described in this universe really does preclude a "good-aligned Mummy". If there's going to be a good-aligned Mummy, there should be a good story to go with that, to say how that happened contrary to the usual Mummy creation process, that the PCs have at least been given hints about. Otherwise, yeah, it's pretty appropriate for a PC to automatically kill any Mummy he comes across on sight. They will know the stories....
Note that the 5e MM does say (page 7 if need a reference for your rules lawyer):
The alignment specified in a monster's stat block is the default. Feel free to depart from it and change a monster's alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there's nothing stopping you".
However, unless there is a good story behind the anomalous alignment, and your PCs have access to clues about that story, I think it would usually be better (and annoy your players less) if you either make up a new monster that isn't in the MM, or be clear that you are playing in a non-canonical setting and using monsters that don't match the descriptions in the MM. Even in a canonical setting, you can play variations on less-legendary monsters, but be clear (out of character) with your players that this is what you are doing. In all cases, allow the players relevant checks to recall some in-game, in-setting lore about the monster you are actually playing.
For example instead of just putting in a good-aligned Mummy you could say, "You see a medium-sized humanoid, wrapped in bandages. Make a religion check". Tell everyone with a low score that they think it's a Mummy. Tell whoever got the highest check, "Because of [some detail that they can perceive] you think this might not be a true Mummy but rather a Pseudo-Mummy. Pseudo-Mummies are created by a different process than True Mummies and in some cases can maintain their pre-death alignment." If you want, you can go into the process more, or you can just say that the character doesn't know any more than that. Now you have a good-aligned Mummy that your player shouldn't complain about.
4) Consider having a talk with the players about what game everyone wants to play. You have a conflict in play style with the "rules lawyer" player. Do the others also want to play "his" game, or do they prefer your approach? Can whoever is in the minority live with adjusting their expectations to what the group as a whole prefers? Can there be some compromise?
Start small — give your players one specific problem to solve
It seems the main issue is:
they don't take initiative AT ALL no matter how I try to make them. I have to ask "What do you do ?" every 5 minutes and most of the time they say they don't know or very vague stuff like they just wanna get the story going
Naturally, when you don't contribute to the story, you have no reasons for role-playing.
In other words, your players lack agency. Perhaps they treat D&D as a computer game, where the story is pre-written, and all you need to do is to make choices (select one from a pre-written list) and participate in combat scenes. So they expect the DM will lead the story, giving them minor choices sometimes.
Instead, let them choose the whole approach. Don't make the adventure too open-ended — for new players it might be paralyzing — but propose a decent task (say, to save hostages from a castle) and let your players decide all the details. How do they do that? Will they choose a sneaky infiltration? Will they try to deceive the guards? Will they make a forceful approach? It is up to them.
Best Answer
There are other benefits to capturing an enemy besides just information
Rather than every NPC either having secrets or nothing, consider other things for your NPCs to offer the party.
As an example, perhaps this particular NPC enemy was just a mercenary for hire and has no real loyalty to the villain. Perhaps they'll leave with their lives if you let them live, not following the villain anymore, but if you offer to pay them, they might join as an allied NPC (although you don't want to let this happen too often, otherwise your players will want to amass an army of cannon fodder, but it's an example of an NPC who is useful for reasons besides info).
Silent NPCs?
Also, I must question the use of "silent NPCs". This implies that you haven't really thought about why that particular NPC is there, what their motivations are for helping the villain. If it's just another cookie-cutter loyal cultist, then fine, just have them ramble about how their time has come and how the PCs won't live to see it, etc, etc, just kill them. But for other, more rational NPCs...
Not every NPC needs to have a rich personality with a huge backstory or anything, but they should at least have the basics of a personality, even if it's just one simple trait and a reason for being there. The DMG has a whole chapter on creating NPCs, with some advice on "Quick NPCs" and tables to roll on to quickly generate on-the-fly personalities (pages 89-91). You can use this to quickly whip up a personality, maybe even only after they've been captured, but since it's quick, ideally before. This might also influence their decisions before they're captured, since not every enemy wants to fight to the death.
Surpriser provided in a comment an account of how successful this can be and how interesting the outcomes it leads to can be:
I think this nicely illustrates the potential for a little characterisation of NPCs and how that can help to enrich the story being told.
Related: What are the stages of influencing Hostile NPCs?