Pcunite,
Welcome aboard.
You are asking a question that is probably on the minds of many folks considering the same move. For the most part, MF is going to require a fair bit of light to shoot at the smaller apertures you are asking about. One way around that is to use wider angle lenses for more DOF, and then deal with any distortion issues you may face. This may be fine for the environmental settings, but may not look as good for more formal portraits, unless you can get the lights closed enough and softened enough for use with a longer lens.
The Canon 580EX II flashes put out a fair amount of light. I have them and use them myself. My issue with them is that it really hard to get a softened light that is able to wrap around the subject a bit more. You may not need or want that effect. (Your shoreline portraits are an example.....good overall lighting that is nicely balanced, but still some harsher shadows from the point sources of the 580s, versus shooting with a larger umbrella or softbox. Just my observation and not a criticism. You have a great light balance in these shots. The smaller flash are just a bit less forgiving in harshness.)
Your interior shots look nicely balanced, and moving the smaller flash units around will be easier for sure. Personally, I have not been able to get the same sort of lighting success with mine as I do with flash heads, packs, softboxes and other modifiers, so I haul more of the stuff around. This is a constant source of some frustration, since I would rather use less and smaller lighting, but have not found it as easy to adjust and control. Your are getting some nice results, so using the smaller units with your 35mm set-up may be all you need. That will give you more DOF options also.
Not sure what you mean by "contrived", but am assuming it to be more studio related for things, such as dramatic angles, background, hairlights, spots, etc. Much of that is a lot harder to achieve with the smaller flash units simply because there are fewer modifiers made to fit them, compare to studio flash equipment.
Not sure that is of much help. I refuse to part with the variety of kits that I have assembled, such as multiple small units, heads and packs, cool fluorescents, etc., simply because there are situations where only one just cannot do the job.
LJ