When I select "Collapse All", I want to see all my timeline's dates at once (with scrolling if required of course) and not paginated with 3 dates (or even 1 date, depending on number of daily messages) and a lot of free screen space per page.

I understand that this behavior is caused by decision to show 100 messages per page. My suggestion would be not to use pagination at all and load content dynamically with scrolling if required.