I'm finding it hard to understand this, where is this modified val being used? next_event_at_age is being updated in the else blocks, and then the QuiesceDbManager::quiesce_db_thread_main() doesn't make use of it anywhere, am i missing something?

another (dumb) question, if the next_event_at_age is already max(), we're adding some duration to it here?

next_event_at_age is used here and here. It will cause the main thread to make a pass at or after the designated age.

This var is initialized with max() to denote "no time-based events are pending", which will make the main thread sleep until the submit condition is triggered.
However, in the body of the main thread loop the next_event_at_age is updated with the output of the various subroutines, e.g. at the line you commented on.

If the db thread has some work, and the boostrap_interval is non-zero, what happens in that case?