//1. If in any case, my job doesn't allow me to take out time (maybe because of working hours) to prepare, will that be a good option to leave that job before a few months of the exams?? Will it mark a wrong impression?//
Thousands of prospective applicants will be in a similar situation. For most of them, their regular jobs will leave them with little time to study after work. Most are going to make the best use of their weekends to make up for "lost" time during weekdays, though some are going to take time off work, along the lines of what you are thinking.
If you take up a job that requires you to work late nights + weekends, you won't have much of an option - you will need to take some time off right before your exams. However, for most applicants, weekends + a few hours during weekdays is enough.
Also, by the time exams are around the corner, you would be in your job for quite a few months, maybe close to a year. By then, you will have accumulated quite a few leaves that you can take right before your exams. That way you can hang on to your job (no "gap" in your work experience) and therefore no "wrong impression" worries.
//2. What if i choose a teaching job instead after my graduation, in a college/or coaching/or institute......(teaching would be on my stream of engineering). Will that be considered as a work experience?//
Though it will be considered as "work experience", unless you can build a great story (for the purpose of the final interview before admission) around why you were motivated to take up teaching as a full time job right after your BTech (which I think is quite an uphill task), this would not be the right thing to do.
//3. Even if i m working, how far is my organization will support me if I want to pursue MBA after a few years of job (and i choose to work for the same company after that) By support, i mean granting me a leave on sat and sun......or some other steps to reduce my working hours. Because they also will be requiring experienced employees with an MBA degree.//
If I understand your right, I don't think you coming back to work for them after your MBA is going to be a strong enough bargaining chip for them to give you extra time off. True they will require experienced employees with MBAs, but then they would have enough MBAs to pick and choose from.
I am not being able to follow your "...mean granting me a leave on sat and sun..." point. Aren't most companies off on at least Sundays, if not on both Saturdays and Sundays? Sometimes there might be fires to be doused, that might require you to work during a weekend or two, but most of them should be free anyway.
//4.lastly, are these things also come under a 'work experience'??
a. Internship for a few months before a job.//
Based on my experience, most companies don't consider internships as "real" work experience, though they will probably give it some weight. Of course some companies do - depends.
//b. i m also working with an NGO and we we organize various events for social cause. I also go to teach slum children every sat-sun.//
This is good to have on your resume - more like icing on the cake; the cake is primary though.
Conclusion: If I were you, I would join my job, and plan as I go along, and not give as much thought right now on how to handle MBA prep in parallel. Things will sort themselves out with time.
All the best.