(1) It is correct.
(2) Once the buyer exited his position by selling it off, his involvement from the position is over.
(3) If the buyer waited until expiry and the premium at expiry is higher that what the buyer had paid, then he will make a profit. This profit will be given by the seller (so obviously the seller has made a loss in this case).
(4) I am not sure if I understood this correctly, but let me tell you this:
If TCS spot price is 2700 at expiry and you had bought 2500CE then your Call option has expired 200 points in-the-money (LTP will surely not be 25 for liquid scripts like TCS ). Assuming you had bought the 2500CE at a premium of 30 points some days ago, your net profit without taking into consideration taxes/brokerage/other charges will be 100x(200-30) = 17000 where lot size is 100.
(5) If you are reffering to Open Interest, that is, the number of option contracts that are currently open then yes. This is because for any Option trade to take place, one buyer needs one seller to take an opposite view to enter into the trade. So for every buyer who bought there is a seller who sold, and vice-versa.
But if you are reffering to no. of buyers and sellers who are yet to buy/sell but are willing to do so(bidders), then that ratio of buyers and sellers may not be equal. You may notice this in the order book that sometimes no. of buyer are more than sellers and vice-versa.
(6) Settlement upon expiry will be done among buyers and sellers whose positions are already open and that will always be equal in number as mentioned in point (5).
(7) Well, at that time it’s slightly out-of-money but some time value would be left in the premium. It’s impossible to pin point exact value as there could be many factors prevailing at that moment in time. Theoretically, as per Black-Scholes model the premium would be around 6 assuming IV of 20%.