"Rainbow" poplar is just regular yellow/Tulip poplar that is stained from minerals in the soil it was growing in. There is no way to fully assure you...
That's the style I have...the previous owners of our previous property were school teachers and left one in the building that became my shop. It now...
Actually, at our old property, we had two units; one for the "original" portion of the house (kitchen, powder room and original master bath) and one...
If it's not dried, stack and sticker where there is good prevailing air movement with just a cover on top so that precipitation doesn't stand on top...
"Technically" the faced fiberglass needs to be covered due to fire risk, so I'm glad you intend to do that with the 'rock or T&G. You might consider...