(1) No, if you want to affect old leads (in the past) set up a batch campaign and change their score. Then for future status changes, use your trigger campaign. ( when I first got Marketo, we had to create 2 smart campaigns, update the past (batch) and the future (Trigger).)
(2) Only batch campaigns (no triggers) give you that visibility before hitting the run button.
If you want that visiblity just for a number count, run a quick smart list.