That’s true, though since it is a reversing refrigerant flow system, it can act as a high efficiency heater as well. The high efficiency heating+ multifunction of the unit ( which negates the need for a separate electric resistive or fuel heater) make it’s overall performance more efficient in both energy use and spatial consumption, doesn’t it? It’s more efficient!
If I’m being a cheap ass but also worried about fire I would get fire resistant big stuff and fill it up. may have to wait till one layer cures to close the gap enough for a second layer and then cut some sheet metal and make a crappy escutcheon type of thing to slap on top of it and call it a day
I’m assuming there’s a rafter tail above the OSB that’s above the soffit panel. So even if you cut that OSB you’ll have the rafter tail to deal with. How does the roof meet the exterior wall above there? Is there flashing? Could that be where the water is intruding? The sheathing and foam was installed before the siding trim and gutters were there so it’s hard to imagine you getting any easy access unless you’re willing to cut a lot of things off and re build them.