My STBX is asking for spousal maintenance. If I were to give her this and capitalise it, she would be unable to claim universal credit because this would count as savings. Am I able to pay her additional child maintenance instead? (I believe this is not included in universal credit assessment). Her income is low enough for her to receive about £300 universal credit per month. If so, would her receipt of universal credit be included in calculations to ascertain if her needs are met?
Yes if her needs can be met by state benefits that she is entitled to then these are taken into account.
If she chooses not to claim them then that is her decision.
One thing to bear in mind if you agreed a rate of child maintenance in excess of what the CMS amount would be it wouldn't reduce the amount of Universal Credit she was entitled to as SM would but once the court order had been in place for a year you could apply to CMS instead. This would override and end the court order and reduce CM. That may make them reluctant to accept a larger CM figure in lieu of SM