> The D” (D double prime) layer, the lowermost zone of the mantle, was describe to me in grad school as the “subducting slab graveyard”.

It’s still fairly unexplained what exactly the D” prime layer is — whether it’s made from a build up of old semi-molten slabs or if it’s even compositionally different from the rest of the lower mantle at all is not yet settled. Even the idea of what the whole lower mantle in general is composed has evolved a lot since the discovery of the D” layer; in part due to new types of high pressure minerals being proposed as important parts of the mineralogy but also due to the ever increasing heterogeneity of the mantle as it gets probed at slightly higher resolutions with time.

>This layer was also hypothesized to insulate the core enough to cause heat anomalies large enough to create break thru hotspots in some places that give rise to features like the Hawaiian or Yellowstone hotspots.

I’m not sure that a slab-graveyard interpretation of the D” layer would provide thermal insulation at all — subducted slabs are colder than surrounding mantle material, even by the time they reach that depth; this would have the effect of increasing the thermal gradient (and thus heat loss) at the core mantle boundary rather than insulating; though in a roundabout way this can cause Rayleigh-Taylor instabilities (ie. thermally buoyant regions) elsewhere at the core-mantle-boundary. Seismic tomography makes a convincing case that the Hawaiian hot-spot has origins at the core-mantle-boundary, possibly from such a mechanism (or maybe because the physics of the fluid outer core just happen to create hotter and ‘colder’ regions of the CMB). The origins of the Yellowstone Hotspot are even more enigmatic, seismic methods employed by Yuan & Dueker, 2005 traces what is likely the Yellowstone plume down to only 500 km depth (over 2000 km higher than the CMB). Either a lower mantle counterpart to this plume existed in the past but doesn’t today, or the origin was/is at some point in the upper mantle.

It looks increasingly like the two huge continent scale structures known as LLSVPs which rise up from either side of the CMB and extend hundreds of kilometres through the mantle could be providing the sort of insulating process that you describe — whereby rising plumes get temporarily stuck underneath them and build up heat and/or material before leaking around the LLSVP edges to continue towards the surface. The whole thrust of the research from Torsvik et al, 2006 was establishing how surface expressions (in particular large igneous provinces) of plumes can be traced back to the margins of LLSVPs. The Yellowstone plume does not fit in with this model, but then that would make sense with it not having a deep mantle origin as origins at or near the CMB would be the ones to get ‘stuck’ underneath LLSVPs.

> Yep. If fossiliferous rocks are subducted, the fossils will be buried in the mantle until eventually, those fossils are somehow transformed beyond being recognizable as fossils (i.e. they're mixed enough, melted or not).

Your use of ‘eventually’ is kinda misleading here. Any fossiliferous rocks would be at the top of a subducting slab and so if they didn’t get scraped off onto the overriding plate they would already be sheared upon entering the mantle; not to mention right at the slab-mantle interface where it won’t be long at all before the heat finishes them off.