Hi there, hopefully I can help with at least the main question :
Although I am not offering any solution I am allowing myself to comment on your other points 
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This is a tricky one. On one side Formlabs is being carefull not turning Preform into a disorganized feature-creeped piece of software which is a mindset I couldn’t be more OK with… This mindset places the limit to the things you can do with a model to what a slicer, just like a CAM software, is supposed to do : transform a 3D model into G-code. With this in mind, offering ways to alter the model isn’t in Formlab’s tasklist at all, and I personally agree with that. In your case, the argument could be that if you are spending a lot of time orienting and setting up the supports it would make sense to be able to mirror it.
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Couldn’t agree more `although when compared with other potential improvement this is pretty far down the list.
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I’m not sure how that would work because I would imagine that Preform would take into account previous support placement to decide where to place the ones higher up on the print.
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It does eat up all the ressources available but does it mean it’s inefficient ? I see it more like it’s optimized to use the full capability of multi-core CPUs which is not often the case even these days. One thing that would be useful for some would be a setting to force Preform to use only 80% or so of the CPU bandwidth, in the meantime you can do that from the task manager by reducing the number of Cores that preform.exe is able to use.
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Both of these points have been adressed in the past and while I agree with you, Formlab’s word is that they aim for Preform to be easy and intuitive to use for beginners primarily, and people who aren’t used to work with CAD or other 3D modeling softwares seem to find it more intuitive to navigate with a fixed rotation point. The rotation restriction I do not understand how that would make it easier for beginners…