All of that about linguistics is very interesting. I appreciate that you spent the time to write all that out for me haha. Sorry I've taken my time to get back to you as well. I'm not sure I quite understand what is implied by your / mark. Does that mean you can say either 度 or 入面？ Because both are indicating a place/location right? I think I get it. Is the difference between 過 and 咗 that 過 means something like (Yea, I did that) and 咗 is like (succeeded in performing something..)? They both seem to carry the same tense. And then that would be why when you use 咗， 坐低咗 becomes the topic because you are boasting in your success of performing that action..? Hahah, hope I'm didn't over-complicate it.

我會坐返（嗰）架飛機 I'll probably take/ride the plane again sorry my mistake. Only 坐返: take + OBJ(means of transport) + again as you see, 坐 as a transitive does not need a preposition (喺), but not like in english that both ride and ride on are acceptable. 坐+喺+location always means 'sitting in somewhere', or 坐+喺+location/obj+direction, 我坐喺張櫈上面。 And as it is 'the' plane, you may need a determiner. 坐返飛機= take a flight, (in general, not a specific plane) well if what here 'the plane' means a scheduled flight, then yes 我會坐返飛機， but, be careful, 'V+返' carries several different meanings too. 我坐低咗喺地鐵度／地鐵入面 I sat down on the train/MTR : 過 is a experiential aspect marker, so 我喺地鐵度／入面坐低過 = I HAD sat down on the train before, and REALLY DOWN (on the floor?) (that's why i put 喺地鐵度／入面 in front of the verb, as it would be the 'topic' of the sentence)