Hi Doug always interested to hear your view as I am sure you know
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The upper current mass limit seems to be set from what I have read so far.
And as Para two of your post... My original views were that black holes were sort of the hub that controlled the rest of the galaxy but as you suggest and the conclusion I arrived at is their mass would have to be so much greater than even the largest known to date to act in the manner I assumed..that of a "control" over the whole galaxy by direct influence.
The cleaning out thing I have read about can only be in respect of a relatively small region one would think because of the inverse square rule application... so in the middle of the densest region there appears to be an area that the black hole has eaten everything...at least with the big ones.. little ones appear to be still munching away as there is still matter for them to consume... but I must look and see what these cleaned out regions are in size relative to various sized black holes.
If you include attraction in the mix I would imagine it must work much as you say... if you include attraction in the mix that is
as you know I will include it on the one hand ( to understand current thinking) and exclude it on the other to build my Gravity Rain Universe...
I now think it is the dark energy that has the major influence overall... not just because of my gravity rain views but because it seems thats what many who know more than me think...An external pushing force seems to be accepted unless its just my morosophia playing up again.
I think it is this feature of galaxies being held together no via a central hub attraction that raises the prospect of dark matter and dark energy.
But as always still learning.
Great to have your input ..thank you very much for taking the time to post.
alex