To answer your questions:
1/ The preference list is a PREFERENCE list, you should always put the courses in the order that you want most to least want. If you don't do that and instead put your most preferred course as second preference and yet you can get into your first choice, then you will have zero chance of ever being considered for that second course.
Therefore, if you really prefered a Bach of comm over Bach of acc then definitely put comm as first preference because you never know if the clearly in drop or other factors like seas can help you get into the course. If you leave it as second and you're quite sure you will get into your first pref then you will never be considered for Bach of comm.
2/ If you get an offer for your first preference, you will not get an offer for your 2nd,3rd,4th...etc preference. Meaning that if you reject it, you will not get any other offers, even if you're eligible for the other courses. Why? Because vtac assumes your no 1 preference is indeed the course you MOST want to do. Therefore they will not bother giving u more offers.
3/ You may only get more offers if your first offer was not your first preference. So for example, you got your 2nd/3rd/4th...etc preference in the first offer round. This will mean that you may or may not get an offer from a higher preference in the 2nd offer round(when uni may reassess your application/places available for the course and reconsider offering you the place).
4/ Note: Always accept the first round offer even if it's not your first preference because you may not get any other offers in the later rounds. If it's your first preference the you definitely have to accept it because you definitely won't get any more offers.
5/ Of course, if you get your first preference but by that time have changed your mind to another course further down the list then the only option that you will be considered for that course is to apply for an irregular offer.
Generally, how vtac goes about this whole offer process is that they look at your list, go to your first preference, then ask the uni for that course, 'will you accept?' If yes, then woohoo! Congrats, you got your most preferred course. End of story.
If however that uni said no then vtac will simply go to your second preference and ask the uni, 'will you accept?'. If yes then vtac will give u the offer for the preference, if not then vtac will simply repeat the same process again until they get to a preference in which a uni says 'yes, we can offer you a place'.
So make sure you order the list as a preference list.
Hope that helps clear some misunderstandings.
Good luck!