These scenarios are applicable only if Sri Lanka wants to win. From the first test, it looked as if they are playing just for formality and have no interest in getting a result other than draw. If they were really aiming for a win, then they would have asked Bangladesh to follow on. And it was not like the bowlers had 150-200 overs of work. Bangladesh in the first innings was bowled out in just 83 overs, and most of these overs were bowled by spinners (Ramesh Mendis and Praveen Jayawickrama bowling 63 overs in total).
Srilanka can still win this match pretty easily. There are still five sessions left in the match and they have to take nine more wickets in these sessions. the only hope for Bangladesh is to avoid losing wickets and see where they stand by the lunch on the 5th day. If they have wickets in hand at that time, they could take on this chase against srilanka at that moment.