The question mark is an indication of "estimated" duration. If you allow Project to calculate the duration (as in question one) the question mark shows. If you manually enter a duration (question two) the question mark disappears.

You can control the display of the question mark Project 2007 through Tools > Options, scheduling tab and unchecking "Show that tasks have estimated durations" and "New tasks have estimated duration".

In Project 2010 it is controlled through File > Options > Schedule and the option "Show that scheduled tasks have estimated durations" and "New Scheduled tasks have estimated durations." So, for the question marks appearing or not, that is an option for each product.

To question 2 - you entered a duration, so the "estimated" no longer applies. The Actual finish disappeared because you increased duration. So the task is no longer complete (% complete is less than 100). The change in duration add the increase to "Remaining Duration". Apply the tracking table to see most of the tracking fields.

To question 3 - you entered a duration which was shorter than the actual duration. Project assumed the new duration is correct, the adjusted the "Actual Duration" to the new lower value and recalculated the Actual Finish date.

I hope this helps.
Julie