A
>Service department confirmed a split plastic coolant pipe; they have
>a spare in stock for about $140 ... but there's 4 hours of labor to
>remove-reinstall the fuel tank to replace the pipe. So I ask, should
>I replace the other pipe next to it while we are at it ?

This is a long shot, but is there a plastics fabrication hop near you. 
Certain plastics can be welded using an ultrasonic welder. Heats/fuses the 
plastic with highfrequency. Why did yours crack? Physical abuse, if the 
crack looks like a long oval tear with sharp corners (referred to as a 
fishmouth fissure) then this is due to sustained high internal 
pressure+temp. The plastic creeps (stretches) until it ruptures. Maybe check 
the pressure relief cap function also.  For $15K I'd expect daily 
driveability longterm. Maybe the pipe was a manufacturing defect (thin wall 
section). Lean on the dealer since he said it should last a lifetime!   tim s.