This seems to outline a use case for having active or updating firmware images cached on the MX appliance SSD (in cases were its a model that has storage).
Presumably you're using a single MS120 to manage 2+ HA links. If this isn't the case and you're using separate MS120, this might be overcome by staging upgrades.
Out of curiosity are these switches setup under a different Network than the MX appliance they're sitting in front of?