Is it okay to over-estimate my advance tax

1. Yes its ok to pay higher advance tax. If one pays lesser amount of advance tax then he/she had to bear the interest u/s 234B/C.

2. You will receive refund of the amount over the total tax liability at the time of return filing.

3. Yes, you should take effect of all your business expenses, investment made in personal name for calculating the last installment of advance tax. 

Thank you for answering my questions.

I think I confused you by the bonus question, I should have phrased the bonus question better, sorry about that. My bonus question was not related to advance tax calculation, but to the final tax calculations and whether if I can claim deductions on investments, insurance that I purchase after 31st match. I think the bonus question deserves its own thread, so I will post it there.