Settling can make your score go down. If you want your score to go up, then pay in full, without settling.
Nope. If the account is in default already, paying will not improve your score. The damage is done and will remain for the balance of the 7 year reporting period whether paid, settled, or unpaid. If the accounts are still open, active accounts, it is unlikely that the credit card will accept settlement for less than the full balance. If they do, they will report the account to the credit bureaus as "charge off/settled", a serious negative. Your score will drop.
If the payment is made after reminders and with penalty, the credit score will be affected. Slowly you have to improve.
It will still affect your credit for 7 years.