When I set the parameter index.prefer.titleabbrev to "1" the abbreviated titles are used in the Index and the Table of Contents also.
Found a reply by Bob Stayton to the same issue by Robert Nagle. https://markmail.org/message/ijt4er434oeelzug?q=titleabbrev+list:org%2Eoasis-open%2Elists%2Edocbook-apps
