Retaining complete compatibility for QtQuick 1 while using the QML 2 engine is impossible. Copy QtQuick1 including the QML1 engine from Qt 4.8 into a separate module and remove QtQuick 1 entirely from the QtDeclarative module.
This will also require renaming of class names in the QML2 engine to, e.g. QmlEngine, QmlExpression, etc. to avoid clashes with QtQuick1/QML1