Definitely accept your first offer. Also, yes try for a later-round offer (accepting this offer will NOT prevent you from receiving subsequent offers).
Now i suggest you do exactly what you stated - enter into Arts then transfer into Commerce.
The requirments are here:
http://www.reg.mq.edu.au/undergrad/current/currentstudents/Table 2.pdf
And since the Arts degree is flexible, you may even be able to enrol in some units that will help you graduate with a Commerce degree.
[I assume you want to transfer into Commerce, not the prof. accounting degree stream]
The requirement is: at least 12 credit points with an overall GPA of 2.0 [GPA of 2.0 = pass =)]
So, yes it will not be hard for you to transfer, as long as you pass all your courses in semester 1 (which you should do 4 subjects in - 4 subjects, 3 Credit Points each = 12 credit points). You can then transfer after 1 semester.
I suggest if you want to do this to try and pick some subjects in semester 1 that would be useful for the B Commerce, if you meet the pre-reqs for it.
You can find the degree rules for B Commerce here:
Bachelor of Commerce - 2010 Course Handbook - Macquarie University [and you choose one or 2 majors listed]
As examples of units you might want to enrol in in semester 1:
ACST101 [no prerequisities]
BBA102 [no prerequisities]
ECON111 [no prerequisities]
MKTG101 [no prerequisities]
So yes, Macquarie is excellent in this sense - it is very easy to transfer [and by transfer, i mean internally - you just submit a form into Student Enquiry Services, no application to UAC needed].
Hope that helps.
