Why no. of correct answers is not proportional to the score I get?

Because, there are lots of experimental questions in your real GMAT which don’t get counted in your performance. They appear randomly and a test taker just cannot identify (with 100% confidence) whether the given question is experimental or real.

I know how many questions I answered correctly still I didn’t get the expected marks. how is it so?

You certainly cannot ‘expect (or estimate)’ your final score. You can’t calculate your score based on number of correct answers given. It’s not directly proportional to your score.

What AWA checks? I am made some grammatical errors in my essays but have written solid essay. Would I get less mark?

AWA section checks your ability to analyze the reasoning in a given statement and to assess critical flaws in argument. It also checks your writing ability to have a critical review on certain issue. Grammar plays less important role in AWA, you have verbal section to assess it.

I am not an American and don’t spell words like American English. I am worried that the “computer programme” while analysing my essays will spot my spelling mistakes and evaluate accordingly.

These issues do not matter much. You should be fine with British English spellings.

I heard that a computer program evaluates essays in AWA section.

Yes. A computer program named E-rater, along with a human examiner (/grader) evaluates essays in AWA section.

Why AWA’s marks don’t get evaluated as soon as the exam is over?

AWA consists of essays that get reviewed and evaluated by examiners. Since there are no examiners sitting on the other side of computer to review your essays ‘real time’, it takes time to calculate the score in scale of 6.0

I ran out of time. I chose a random answer but couldn’t hit confirm button for the last question. Will it be considered a blank/ unanswered question or my answer choice would be considered?

Fortunately, your answer choice will be considered. & if it was a correct answer, marks will be added even when you forgot to hit confirm button.

I ran out of time & left question unanswered. Am I going to disqualify or penalize heavily for this?

Leaving last question (/questions ) black is worse than selecting wrong answer choices. At least try to select one random choice first before hitting confirm. GMAT score gets penalized in such cases but it’s not the end of the world. You’re not disqualified.

In AWAs essay I wrote a complete essay but while reviewing, time got up and I couldn’t hitsubmit / confirm button. My all efforts are wasted!
Stay calm, and concentrate on your coming quant & verbal sections. Whatever you’ve written refined or rough on computer is stored and will be taken as respective essay, even when you forgot to hit submit button.

I want to send my GMAT score to b-schools based on my performance on test; Are you allowed to decide five b-schools after the exam?

It’s sequential, you mention 5 b-schools first then you face the test. Sometimes, by mistake if you missed to fill that form, you could get lucky and allow to write 5 schools name after the exam. But do not try this deliberately, it’s risky & you could lose your opportunity of 5 free reports.