6. Yes, the correct letter is an O. The writing is 4,(it could be lengthy in a book or short on a mobile phone), add the usual two-letter abbreviation for 'that is', with the usual abbreviation for Left inside.
4. Yes, one is superfluous - the answer's just a double definition.
I have 19 across and know whatt adjustment needs to be made but am struggling with two four letter words - and calling myself several more because of that. 24 across - See side of field you are to sell - vend, but why - and 28 down - Fifteenth group in competition sport ( redundant letter - o ) - Polo but why. Any pointers would be appreciated.