Fas, I've been (and still am) in a similar situation like yourself. Most likely, this social situation is a consequence of having certain personality traits, so you will have to learn how to deal with it in an adaptive manner since you will probably be in this position the rest of you life.
1. Like DW, my opinion is that lying should be avoided as much as possible. Instead of pretending you don't know, I think it's much better to admit you do know, but you cannot tell since you have promised not to. When you confront the curious person like DW suggest, they should realise that they themselves wouldn't want you to tell other's what they have privately confided to you. You can also use broad, general expressions such as "He is having some difficulties right now, I suggest you ask him if you feel concerned". If your friends don't accept that you keep the integrety of others - to hell with them
2. We all have the ability to manipulate others to a certain extent, especially people who are close to us and are emotionally attached to us. However, what you describe here is an extension of that natural social phenomenon - an ability to possibly influence many more people, perhaps people who don't know you and perhaps to the dangerous extent where exploit and making people do things they will regret later.
A fundamental issue here is the very important distiction between influence and manipulation.
Influence is something open, it could be conscious or unconscious, but it is out in the open, whereas
manipulation is
hidden from the person being manipulated. Either the true goal is hidden, or the methods, or both. I would give you a couple of examples:
I used to have patients in psychotherapy when I worked as a clinician. Psychotherapy is all about helping and supporting a person to change disruptive aspects of himself. I work with Cognitive Behavioural therapy. There is no secret agenda - you make an asssessment and a diagnosis to define the patients' problems, you present to the patient how you are going to work with it, and each and every step you take, you explain to the patient why you are doing this and what the expected outcome should be. Still, it is one of the most powerful ways to influence another person's behaviour, thinking and even emotions.
Another example is when my friends are doing things I strongly dissapprove of, such as displaying self-destructive behaviour (ie taking drugs, staying in a bad relationship or at a bad job). Let's say I have a friend who drink too much. I will explore the reasons why my friend is drinking to much and try to persuade him and support him to change the underlying causes. I will present all the arguments why drinking is bad for him, and I will offer alternative coping strategies that are less destructive for him. In short, I will bombard him with the goal to make him quit drinking - but I will do this
without going behind his back (ie make his girlfriends hide the wine bottles in their home) and without using emotional blackmailing (ie make him feel guilty by telling him how bad I fell because he is drinking, or stating he is such a bad person so I don't want to be friends with him unless he quits drinking).
Both the above are examples of how you can influence people without resorting to manipulation. Typical examples of manipulation on the other hand, include all the kind of rhetorics you see in popular media and from politicans. Appeal to irrational, emotional arguments rather than factual arguments, claiming one thing but having a hidden agenda, making people support things without knowing what they are supporting. However, on a smaller scale you see this between individuals as well. Sometimes manipulation is unintentional or at least unconscious, such as is often seen among people who feel insecure that they can get what they want and people who feel they don't have as much power as they believe they should have. This kind of behaviour is easy to avoid by simply making sure you have a good self-knowledge and self-confidence and also checking you own behaviour in situations where you know you are vulnerable. Then you have the intentional manipulation - you can flatter, lie and befriend other people to follow you wishes, but I understand this is what you want to avoid. So if you don't trust yourself you must check yourself by constantly ask yourself:
A) If you really mean something genuinly or not
B) "Theory of mind"-questions and if you genunily want the consequences of these or not.
"Theory of mind" is how we think other people will respond in terms of emotions and intentions. So let's say that you can't trust yourself when it comes to attractive, easily manipulated girls. Then you apply the above by:
1. Before saying compliments, ask yourself whether you really mean it or if it is merely politeness. Or if you do it because you feel good when the girls respond positively to your compliments.
2. Use the "theory of mind" view: how do you think the other person will respond to your behaviour? Let's say you really think this girl is pretty and intelligent. But if you say so, she might think that you are hitting on her. Do you really want her to get the impression that you hit on her? If she thinks so, what are you going to do then? Assess the situation, and decide what consequences you are prepared to face and how.
The ability to influence other people is something that can be used for very constructive purposes. I think it should be used in order to influence people to form their own opionions based on facts and rational arguments, to make their own informed choices and to be critical.
I would recommend to only use manipulation skills as a last resort in extreme situations, such as if you have a friend who is stuck in an abusive relationship, drug addiction or caught in a destructive cult, and all fair ways to influence the person has failed.