Why is market power presumably high when goods are fungible and demand relatively inelastic? What does the fact that the goods in an industry are fungible add to a market power analysis if you already know demand is relatively inelastic?
"Fungibility", when referring to goods, essentially means homogeneity -all units of the good are exactly the same from all aspects that matter to consumers (including "exterior" aspects, like location etc), and so one can substitute for another. 

If one knows that in a market essentially only one homogeneous good is traded, then one knows that he must look for market-power sources elsewhere and not in product differentiation (since the latter is non-existent), like for example in barriers to entry, cost structure etc. 

If "fungibility" is narrowed down to the inherent characteristics of the good, then geographical location or other such aspects, may become possible sources of product-differentiation, and so of market power.

Fungibility is a property coming from the supply side, so by construction is unrelated to the elasticity of demand (which depends on preferences). Therefore it is a useful piece of information on its own.