how to alter "other" field label or change the label name How to alter "other" field label or change the label name
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How to constrain the sum of these responses to be 100?
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For customizing the 'more items' see Issue #2953803: Allow multiple value elements item(s) label to be customized.

For 'How to constrain the sum of these responses to be 100?' you are going to have to write custom client-side and server-side validation code.