Your idea of "inherent to the concept" is VASTLY different than mine. In fact, in all the complaining about this spell in 5e, this is the first I've heard of this interpretation. Forcecage has always been criticized due to being a 'sure thing,' no save (or escape via teleportation) way to eliminate a foe from the fight, then gather round and administer a beat down when your are ready and the timing is right. Or, for the barred version, just gather round and shoot through the bars at the nearly helpless victim.
Spells do what they say they do, and while the 'plain English' method relied upon by the devs in 5e certainly has its faults and interpretation issues, suffocating someone with Forcecage is a bit beyond the pale and surely would be mentioned if that was the intended effect. I thought we had left this kind of hyper-interpretation of spells behind editions ago with "I cast create water...in his lungs." Or "I create a Wall of Force Edge on so it creates a razor like implement that is neigh on invisible that they will run into and mangle themselves on." Inventive, no doubt, but somewhat tiresome.
As to the fireball example, yes, you are correct. The fireball impacting on the bars of the cage would then grant the incarcerated target, in my ruling, 3/4 cover (+5 bounus to the Dex ST). Not much, but the best I could come up with to help tone down the spell without outright house ruling it; though house ruling might be a better option.
