So this probably isn't the answer you want to hear...
First I'll just point out that the rest of my post is from the perspective of wanting to be a quant, i.e. I won't be commenting much on your prospects of entering asset management or IB upon your graduation after 4 years (i.e. without an MFE). Only point I have to say there is that your university background for IB is nowhere near as important as your knowledge of recent big deals and macroeconomic conditions (and being able to form opinions on transactions). Not too sure about asset management.
If you've researched about MFEs then you probably know that an MFE qualification doesn't stand much of a chance against a mathematics PhD (can be a financial mathematics PhD) for wanting to be a quant if other achievements are equal. In the same vein, my main advice is to
study as much maths as possible. Take a look at the
recommended book list on Mark Joshi's website for example. You will not be exposed to the levels (or fields) of mathematics that are present in the analysis or probability theory books on that page if you stick with the study plan options you posted above. You probably know that Mark Joshi happens to be the Director of the Centre for Actuarial Studies at the university and that he has a wealth of experience in the quantitative finance industry, so his word is reliable (or, at the very least, believable).
Having come from a very similar background as you at university (BCom in Actuarial) and also seriously looking at the quant finance career path, the first thing I can tell you is that I feel I have an inadequate background in maths. I took on a concurrent maths diploma in undergrad, so I have more maths subjects than you but less Finance/Economics. I can definitely say my competence in complex analysis and differential equations is not up to scratch. Whether you feel that these are important for being a quant is up to you, but if you read books on quantitative finance then Fourier or partial differential equation techniques are not uncommon. (For asset management and IB, the story is obviously going to be different, since the mathematics requirement is just not the same.) I didn't study most of the Finance/Economics subjects you have listed, but for a quant finance career I don't think it is much of an obstacle in the way of quant ambitions.
I would absolutely say that you should just switch entirely to a BSc (despite you saying you don't want to waste your first year at uni), were it not for the fact that this could present problems for enrolling in Advanced Financial Mathematics ll. This is my second piece of advice: If you are going to be completing a significant amount of your tertiary study at UoM,
do whatever is in your ability to be able to enrol in Advanced Financial Mathematics ll.
Advanced Financial Mathematics ll has in recent years been taught by Mark Joshi himself (and I see no indication of this changing any time in the near future). This is essentially
the quant subject offered at the university, in the sense that you will be exposed to an amazing mix of pragmatic and theoretical knowledge of quantitative finance. I am currently studying this subject, and I am thoroughly enjoying it. I did not satisfy the official prerequisites, but I did study subjects which Mark deemed sufficient as background. I take this to mean that you will be able to enrol if you have the sufficient background as judged by Mark (rather than strictly Advanced Financial Mathematics I as is listed in the handbook). For example, the main reason I was able to enrol was because of a Masters-level maths subject Stochastic Calculus with Applications. Most people qualify for AFM2 via AFM1, so these are people enrolled in an Honours year in actuarial studies or an MCom in Actuarial Science (I am not one of them).
Putting together my two pieces of advice, I would say you do this:
Enrol in a concurrent maths diploma, and do not cross-credit any maths subjects. This is the easiest way to study as much maths as possible, while maintaining eligibility for an Honours year, or rather, AFM2. In fact, this really allows you to preserve most of your planned subject choices at the expense of a later graduation. In my opinion, an extra one or two years at uni is just not a material problem in the grand scheme of things, unless you have some serious financial considerations.
In terms of the specialisation for the diploma, I would suggest one of
The specialisation is just a formality, because what you really need to care about are your subject choices for the diploma.
Essential
Highly recommended
Might be nice
I haven't discussed any of the logistic issues with fitting these subjects into a study plan.
Apologies for this not being the answer you were probably wanting, but I hope it helps in some way.