1. The information on page 13 of the notes states that the document you use to prove that you're in receipt of benefit must be no more than 3 months old. So it seems that you'll have to try to get a new letter.
2. From your previous post it seems unlikely that you're living with your children's father so, unless you're married to him, your status is 'Single'. (See page 7 of the notes). Therefore the 'contrary interest' part does not apply.
3. The 'disposable capital threshold' is provided by the table on page 11. As the normal court fee would be £215 (i.e. less than £1000), the disposable capital threshold that you must fill in is thus £3000.
4. You're applying for 'Remission 1' (i.e. full remission of the fee based upon the fact that you're in receipt of a relevant benefit) and not 'Remission 2' (full or partial remission of the fee based solely upon a low income). Only applicants for 'Remission 2' need to provide proof of income (since the fact that you're in receipt of Income Support makes it clear to the court that you've obviously got a low income anyway). So you don't need to submit anything about maintenance payments.