score:1
If your train to the airport has a stop in Brussels-midi you can surely get of the train earlier. You might need to buy an extra ticket for the Brussels-midi to Brussels airport trip a couple hours later. It's a good idea to buy that extra ticket before you get on the train to the airport to evade an extra charge of 12€ or something. But that should be easily doable on the day itself.
Upvote:4
Now, for your specific journey all options in the planner on the website involve a change in Antwerp, not in Brussels. So you will not get away with visiting Brussels. An alternative would be visiting Antwerp. You will get away with that, even if it is strictly speaking not allow to interrupt your journey there.
A ticket to Brussels-Midi would be one to "Zone Brussels", which includes all stations in Brussels. The airport is not in this zone, because the airport is not located in Brussels proper, but just outside of it.
Finally, you will not find anywhere on the website of the SNCB the ticket described as a ticket to Brussels plus surcharge. You will find it described as a ticket to Brussels Airport-Zaventem, as the airport station is called (formerly Brussel-Nationaal-Luchthaven) including the surcharge.