This is an interesting question that is maybe a little misguided. Instead of answering your question directly I'd like to draw your attention to some things that might get you to rethink your reasoning.
It turns out that what you're thinking of as the lower mantle (an assemblage of solid phases: mainly bridgmanite + ferropericlase + some other stuff) might not actually extend to the core-mantle boundary (CMB).
In 1942, a mathematician and geophysicist named Keith Bullen came up with a scheme where he labeled the layers of the Earth alphabetically (the crust is A and the inner core is G). In this scheme, the entire lower mantle was designated as a single layer (D). In 1950 Bullen decided to split the D layer -- the top ~1800 km became D' and the bottom ~ 200 km became D''. The D'' layer is associated with a complicated seismic discontinuity, and geophysicists are not entirely in agreement on how to interpret the discontinuity. However, there is general agreement that the seismic signals from the D'' cannot be explained by assuming that it is simply an extension of the lower mantle to the CMB.
My preferred explanation is that D'' represents a thermochemical boundary layer that includes partial melting. A more in-depth explanation can be found here.
Regarding your question about blobs of iron...It's unlikely that blobs of pure liquid iron would be neutrally buoyant anywhere in the mantle. If you're interested in a more technical explanation of how the density of iron changes with depth (i.e., pressure), you may want to look into finite strain equations of state. The Birch-Murnaghan equation of state is a good place to start.