Hi Andrew!
The answer to your first question is simple, but not necessarily obvious: You must create a separate filtered view of Chords and plug that view into your formula, instead of the table itself.
Why? because RandomItem([some table]) may return any item from the table (regardless of any filters applied) while RandomItem([view of table]) can only return an item this is shown in [view of table].
For your second question, there is no way I know of to get Coda to trigger an action more than once per hour, except there is a tricky workaround. Check out this example doc:
Let me know if you need an explanation of what’s going on here 