Is it safe to cut throught a 70mm spruce beam with a Metabo jigsaw?
Yes, the problems you mention are possible if you try to advance the jigsaw too aggressively and even with thinner materials. Just don't advance it too fast and especially don't force on the jigsaw and it'll be fine although rather slow.

The other side is that those 10cm blades are rather expensive and you might be much better off just using a handsaw for the few cuts you need.