At Length
tenderer industriousness
The oxymoron signals the problem: they mistook death preparation for renewed energy. 'Industriousness' usually means vigor, but 'tenderer' suggests something fragile.
Death lit all the shortness up
Only after death did they see the urgency. 'Lit up' makes death the illuminator—it revealed what was already there.
Carrara guide-post
Carrara marble, from Italy—expensive white stone used for tombstones. The 'guide-post' is literally her grave marker.
So leisurely were we
The final reversal: she was racing to finish living; they had all the time in the world. The rhythm slows down to match 'leisurely.'