Not all of my customers return to the checkout success page after completing payment. Can these transactions still be tracked in my sales funnel visualisation or Adwords conversion goal?
These transactions can be tracked in your sales funnel visualisation by changing the 'goal success' URL to checkout/onepage/PlaceOrderClicked. This will then track checkout success based on everyone who reaches this page (instead of reaching the checkout success page).
These transactions cannot be tracked as part of an Adwords conversion goal.