I also think that toolbar should not change if another tooll is used.
If I would want use cerntain tool and to change toolbar (and continue using it). I would change toolbar first and use the apropriate tool afterwards.
Booxstor, I think that the best would be to implement (user selectable) smart dynamical toolbar (in addition to current toolbars), if Qt library allows that. There would be last 4-5 tools used (in order of usage frequency). It is easy to implement (if Qt allows using dynamical toolbar - changing the icons and actions assigned to them).
And if you add toolbar change freeze/unfreeze (and push/statistic button order) as bonus every need is satisfied.