Suppose you have a function GetCurrentValue which gets the current value of user certain interface objects, and returns missing (.) for others. Suppose you have a list of interface objects (AllInterfaceObjects).
Depends on what kind of values GetCurrentValue returns. Suppose GetCurrentValue returns (checkbox << Get Selected Indices()) for checkbox items. If you apply IsMissing to the list of indices you do not get a boolean literal, you get a list of booleans.
!IsMissing({1,2,3}) = {1,1,1}
Because a list of booleans is not True, Filter Each will remove it! That means FilteredValue will not contain your list, even though your list was not missing.
Question 2
As you may or may not know, Create Database Connection is a function for creating database connections. It returns a database connection on success, and missing on failure.
Does this work?
connection = Create Database Connection(magicString);
if( !Is Missing(connection),
RunSomeQuery(connection),
Print("I have failed to connect")
);