I find your statement that there "may be a very large population" of early black holes that have now grown to "upwards of 1.5 MSols" black holes very interesting.
With much less knowledge than you obviously have, I postulated a dark mass of 2.2MSols was now approaching our solar system in a book I wrote, trying to attract more students, who are now totally uninterested in science, to become more interested in the sciences, by scaring (slight gravitationally induced change in Earth's orbit causes rapid onset ice age - Northern summers too cold to all melt prior winter's ice, etc.) them a little. (I'm a retired physics prof/ researcher.) All the physics in book is valid, but woven into a scary story, and perhaps not "very highly improbable" as I thought, if you are correct. Perhaps only "very improbable"
but most cosmic disasters stories are "quite improbable."
In addition to a small black hole, I also discussed the possibility that this space visitor could be a very old neutron star. Stating that it might currently be undetectable at 130 AU in reflected sunlight, except by the largest telescopes that were all tied up working at high magnification on distant objects /regions of space, because (1) it was no longer a pulsar or (2) was one with residual magnetic now too weak or mag field too well aligned with the spin axis to make detectable EM radiation, and or (3) had axis pointed nearly transversely to its trajectory toward solar system. I noted in book that if it formed with mass just above the max mass for a dwarf, (1.4Msol) it could have grown by slow (did not want it to get detectable hot again) accretion of Hydrogen and "cosmic dust" to the postulated 2.2Msol. From what you say about BHs, this postulated mass gain seems reasonable.
I of course tried to read you mass distribution reference, but find I don't know how. They are not web pages and I don't recognize the journal, if that is what they are. Suggestion, comments please.