So your theory is that solarian crystals can modify the entire dMage if the weapon, but fusions on the crystals can only affect the bonus damage? I can't buy into that one.
If a solarian crystal’s damage entry lists an abbreviation after the damage, however, the additional damage the solar weapon deals is of the indicated type.
I see the argument, considering the fusion is applied to the crystal and not the weapon.
But that sounds like it would seriously hamper the utility of fusions for a solarian. I doubt that's the intended effect of a fusion on a crystal.
Weapon crystals oddly enough are not weapons. You can't just take one in your hands and attack with it. The plus sign in front of the damage they deal in the tables is unique to them. However, its clear they accept weapon fusions. So when a weapon fusion refers to a weapon, it must be refering to the solar weapon, and thus refer to the damage done by said solar weapon in total.
Now, when you take a fire damage adding crystal, and put say a corrosive fusion on it, technically the solar weapon already is dealing two types of damage, some kinetic plus fire. Whether this means you can convert all the kinetic to acid or all the fire, or some mixture of the two to acid, I don't have a definitive answer.
One interpretation leads to solar weapons dealing 3 types of damage in a single hit, which they seemed to try to avoid with all the other weapon fusions, so perhaps developer intent is you compleletly change the kinetic or fire to acid, rather than half of each or even just half of the fire. Thus limiting it to two damage types. Similar logic would then apply to merciful.
Holy should just straight up apply to all the damage.