My goal is to transfer mid-year into either of the four degrees (in stated order):
Keep in mind not all degrees accept mid-year transfers, its definitely something to look into.
I did not however complete the required MATH pre-requisites in year 12 for any of the above courses (ie maths methods).
It is absolutely impossible to get into a degree without doing the prerequisite in VCE or doing its university equivalent. They're prerequisites for a reason, you need to do them to get in, if they let people get in without doing them, then there is little point having them at all. Not at all trying to be harsh but that is the reality of the matter.
You will need to take the methods equivalent subject that RMIT offers and pass it. Once you do, it'll most likely count towards fulfilling the methods requirement to getting in. Keep in mind that the uni version is 2 years of VCE methods compressed into a 12 week semester, i'd say it is harder than VCE methods just due to the lack of time to baby it through you. I did the Monash one and i passed but not by heaps. Many people actually failed it because they go from doing no math in 2 years to doing methods compressed into 12 weeks. I say this not to discourage you but that just simply how it is, it definitely is doable but it is hard for some (cant say what it will be like for you).
Also keep in mind for both kinds of degrees but especially computer science, the math definitely won't go away, it'll always be there. They'll most likely want you to take math more advanced and harder than methods down the track as well. I was once in your position looking at software development/comp sci too and i thought i could deal with the math even though i was barely making it in math, thats simply unrealistic, dont fall into the same trap i did. I ended up changing my mind for other reasons but its definitely something to keep in mind, especially with computer science which is more theoretical and math will leak into many of your other subjects.
As for which subjects can be credited, most ones that are similar to ones at monash or RMIT should be fine, just look through the handbook. Usually they do grant a fair few of them though. If all else fails, contact the university.