With that flexibility comes the chances of having to wait. You are very unlikely to get into MTD before about 7 if you go into the no reservation line, though you should still book something (even if only 7:30 is available) and then adjust on board.
It's like just about any other TA. For round trips, it may not be the least costly. The usual advantage for A2S is one way flights in conjunction with a repo sailing.
The two scenarios reported are it’s either - GTY or <cabin number > after the booking number once barcode is decoded. Anything else I suppose is abnormal.