Agreed. As the sellers of the options are under obligation to fulfill the terms of the contract if the buyer chooses to exercise his rights.
So, the losses of the buyer is limited to the price of premium paid while profits are unlimited, where as for the seller the profit is limited to the premium received while losses are unlimited. thatswhy only premium amount is required to be paid when you are buying options while margin amount similar to what is trade in futures is required when you are shorting an options contract.
The reason for margin being high in shorting is since the upper level of the price can go up to any extend.
If you are writing/shorting an option your profit is limited whereas your loss is unlimited, so that’s the reason the margin for writing/shorting an option is more compare to buying an option.
If you are multiple trades in the same scrip in a day, the average price will be calculated considering the total quantity traded that day. Explained here.