Zee, so very sorry for the loss ... it is hard to lose a good breeder! :grouphug2:
IMHO, the only way it could have been heat stroke was if she was pre-disposed to be succeptible to it ... which the kits aren't displaying any sensitivity to the heat, so I would rule that out as a genetic issue. However, there are other environmental issues that could present with the same symptoms ... like MSD said, and I saw last week when I lost a 2.5 year old doe, there would have been wet fur around her mouth if it was heat stroke, enough you would have noticed plastered/spiky fur even if it had dried.
More than likely, it was a blood clot, or heart failure, or simple fright by a predator. IMHO, go ahead and move all the rest of the buns back outside, keep an eye out for signs of cats/predators disturbing them, and choose a growout doe from her litter if she was giving you the type of kits you want.