If the lower layers are denser despite their much higher temperature and also super viscous, how can volcanos ever form? What if we drill right through this area with an even later size hole? In this caw tho, note that all of the back-pressure is missing at the hole, so gases would violently went to escape (10’000s of bar vs. 1 bar), which should be plenty of driving force to get stuff above the ground. Due to the outgassing the density should also massively decrease. Kind of like lowering a pipe into a volcanic lake that has CO2 dissolved in the deeper layers and letting that vent in a fountain.
you can start creating small steam bubbles within that will continue to grow as the rock ascends, further increasing the pressure (like a bubbly bottle of champagne).
Note that the pressure would go down, not up, as it ascends. But the volume would go up because of the lower pressure.