Good idea to have multiple steps. However, it only scrolls downward then gets stuck at the bottom. Another IF would fix that.
Here is a script that will scroll down the sheets one by one, moving the focus to the sheet. When at the last sheet, it will take you to the top sheet. Sorry about the poor formatting.
tell application "System Events" to tell application process "Numbers"
tell outline 1 of scroll area 1 of splitter group 1 of splitter group 1 of front window
-- Find the sheet that is currently being displayed
set currentSheetRow to first row where ((value of attribute "AXDisclosureLevel" is 0) and (value of attribute "AXSelected" is 1))
-- Try to move to the next sheet. If at thebottom, go to the first sheet
try
set targetSheetRow to first row where ((value of attribute "AXDisclosureLevel" is 0) and (value of attribute "AXSelected" is 0) and (value of attribute "AXIndex" is greater than (value of attribute "AXIndex" of currentSheetRow)))
on error
set targetSheetRow to first row where (value of attribute "AXDisclosureLevel" is 0)
end try
tell targetSheetRow to set {value of attribute "AXSelected", value of attribute "AXDisclosing"} to {true, true}
-- Focus the sheet after the target row is selected.
set value of attribute "AXFocused" to true
end tell
end tell