Half Bridge Inverter Effect of Load on waveforms
First, one misconception to clear up. You said:


  as the Vs (which should be a square wave) massively droops and therefore the Vho is too small to turn on the high side MOSFET for the normal time expected.


However, you have a boot-strapped driver. It maintains a Vgs of about 15V (from VCC) at the gate. If you plotted both v(ho) and v(s) on the same graph, or (v(ho) - v(s)), you'll see the 15V is maintained when the gate is supposed to be on. Put another way, v(ho) looks weird because v(s) looks weird.

Which gets to the first problem with the 100µH inductor simulation: your currents (i(L)) vastly exceed what C1 and C3 can supply to keep v(va) near mid Vrail. Take a look at v(va) and it'll probably be swinging wildly around. 

The second problem, which Tony mentioned, is the Rdson of your MOSFET. Looking at the part datasheet, it looks like it is about 0.25Ω nominal. Based on your inductor currents, it's one of the culprits for the crazy v(s) waveform. 50A at 0.25Ω Rdson gives 12.5V drain-source drop, which is dangerously approaching saturation in this device. The Rdson also explains what you call "droop" in the second simulation.