First, I believe you are overreacting to the current situation, all I'm aware of is the disappearance of flight MH370, and I believe that will eventually prove to have a tragic but non-malicious explanation, with an accident, heroic pilots (and maybe a touch of government bureaucratic incompetency thrown-in during the search, not that it would have probably mattered).
But that's one flight, there are tens of thousands of flights a day, the chances that you will be on a flight that has a serious incident are minuscule and even less chance that you'll be injured on such a flight.
With that said, I'll answer your questions.
What should you do if: The flight seems to be going the wrong way, i.e. a totally different path?
Nothing. You are almost certainly wrong (direction, etc, can be difficult to judge in flight with no landmarks) and if you're not there's is almost certainly a very good reason for what is happening. The passengers will be informed when it is possible to do so, although that's not likely to be immediately. You can, politely, ask the cabin crew if you're worried but they may know no more than you. Doing anything rash may cause other people to think you're up to something, or incite panic in the plane, or generally create a situation you want to avoid.
What should you do if: It sounds like something terrible is happening inside the cockpit?
Nothing. Probably. Buckle your seat belt. On the off chance the cabin crew haven't noticed then inform them. You'll not be able to get into the cockpit, and I'm reasonably sure unless there's explosion or gunfire you wouldn't hear a commotion from the cockpit anyway. After that follow instructions from the cabin crew. Trying to access the cockpit without being explicitly told to do so is going to get you in more trouble and potential cause more reaction. If there is something going on you're likely to distract the pilots and the crew from dealing with it.
Note, if you're a doctor or coincidentally a pilot rated to fly the current aircraft it might be wise to let the cabin crew know that when the opportunity arises. Just in case. Although if they need one of either they'll ask (or they'll already know).
What should you do if: Someone amongst the passengers is trying to hijack the flight using a weapon, whether chemical, gun or bomb etc.
Probably nothing. This really depends on the circumstances, the advice used to be to comply. Most hijackings landed and were resolved on the ground (although not always bloodlessly), doing anything in the air was a much bigger risk. Pilots and crew would have followed this rule.
However after 9/11 it's clear than some hijackings have no chance of a positive resolution and at that point you may want to consider acting. However, you have to ask yourself -- am I absolutely sure I understand what's going on? do I have the ability / capability to do anything? is someone else more qualified (i.e. an undisclosed air marshal) waiting for an opportunity to do something?
I recall reading some advice somewhere, but I can't remember where, but basically if you are absolutely positive that the flight has been hijacked and is not landing in one piece then you should start trying to work together with other passengers and crew, pass notes, use hand signals, coordinate, assess the situation and attempt to overpower the hijackers as one force. Of course hijackers also know this which is why it's fairly unlikely they'll try that again unless they have someway to subdue the passengers.
If the guy beside you is trying to light his shoe or underwear on fire then absolutely you should stop him. I also doubt terrorists will try that again.
Again, unless you're absolutely positive that there's no chance of getting out of the situation alive you should comply and go along with the demands. Fighting with people with guns, bombs or chemicals at 30,000ft is a good way to make a bad situation worse.
But let me re-iterate my first point -- the chance of this happening to you (or anyone) is very small indeed. You can see some US stats here, I can't find a good link for global statistics, but you're more likely to get hit by lightening than die in a plane crash.