I would the option in the Smart Folder to show...
-Ticked (user selected) and therefore automatically synced, sub folders that are
Can you please explain your needs more specific:
a) ticked (user selected)
where and what exactly you want to tick.
b) which folders do you want to synchronize?
c) subfolders: are you talking about IMAP or / Exchange accounts?
I would the option in the Smart Folder to show...
...
-Read or unread (user selectable)
...
before they drop out of the Smart folder
You can use the option "mark read/unread" by using the already existing icon in the action bar - or by defining the corresponding swipe action
I would the option in the Smart Folder to show...
...
-For a given age, (again, user selectable). Ie. 2 days and marked as read.
before they drop out of the Smart folder
You are already able to define which messages shall be shown in the Smart Folder:
Settings > Smart Folder > Smart Folder time period > choose one of the offered options.
(see attached screenshot)
...
Unless they are starred in which case they stay indefinitely.
Furthermore you are able to specify that starred messages should be always shown in the Smart Folder - independently which time period you have chosen