Hello and welcome,
To begin with, think of a conflict as a series of tests rolled up into one big event. However, we’re not rolling for every swing of the blade. Instead, think of beats of a battle scene in a movie. The camera moves around while the battle unfolds.
Maybe a play through based on the Balin’s Tomb scene for LotR might help:
Caveats: this is an extreme example to illustrate a point about bosses, it was written long ago for 1e, some of the rules have changed slightly, normally you don’t have so many characters involved, I did not write out all of the help for each character but players need to describe what they are doing for each action.
But, to your question, you can see how the entire scene unfolds very cinematically. It is not a simulationist, blow-for-blow, literal attack.
1) Should a Maneuver action be a must before any close combat attack?
No, that is not the procedure for a normal conflict. However, Warfare conflicts in the LMM book have a similar mechanic to engage a unit. For a normal conflict, narrative positioning does become important for helper actions and ultimately compromises, but it is all abstracted. The players description needs to make sense though, so as the GM you can help them by providing vivid details and detailing the scene.
2) Maybe the maneuver is not needed and the attack action already implies the movement forward
Think of an Attack action as “something that advances the adventurers toward their goal” rather than a literal attack. So, yes, attacks advance and move you toward whatever it is you are doing. The book provides some good insight into how to translate an Attack for the other conflict types. So an Attack in a Flee conflict means you are running away (moving toward your goal).
3) where is the advantage of missile weapons
There are lots of benefits. The bow basically turns one Attack/Attack into a versus test once per conflict. You get to use the bow once for this purpose so long as the fiction allows it. For example, you would not be able to use a bow at all given a situation where room would not allow or narratively the party was already in melee. Besides that, those weapons ignore leather armor and have a few other modifiers.
4) How do you DMs manage the compromises a winning party
Compromises are a negotiation with the players. The losing party begins. If what they offer is too little, the GM can point to the book and counter with a more appropriate offer. It is important to set the bar, but the rule book offers suggestions (with a few exceptions for warfare conflicts and Killing is My Business). But overall, a compromise is a matter of coming to terms together.
5) Do you really kill characters that easy?
Death does not come easily in this game. Death is a choice: a life for a life. Death should not happen accidentally, but nor should it be entirely in the hands of the players–that is to say that sometimes, through a twist, the party might find themselves with limited options with no way out except to fight.
When death is on the line, the GM is obligated to alert the players. This is the important part of the player agency. The players make a choice to raise the stakes to the highest level.
6) you do it by chance among 0 hit points PCs?
When determining a compromise, that is one way to do it. The compromise (and everything in the game) should flow from the narrative. It is always description + mechanics together. So, if the dwarf got knocked out and there was a major compromise owed, then certainly injury would be on the table for that character. However, the players can counter and offer something equivalent instead. In my experience, they often will come up with ideas that are even more harmful than anything the GM could have come up with.
So, overall, Torchbearer greatly benefits from abstraction which allows the players’ imagination to run wild.