Okay, so no one knew about limestone hearths...
Turns out that the hearth being supplied in 3 slabs is normal for materials like granite and marble. I've been able to check with the manufacturer who assures me that because the hearth is a 2" and solid, and limestone is a soft, using a solid hearth for wood/coal burning fires is acceptable. If nothing else I have it in writing from them, so if it does crack, I can go and make loud noises at them....