"the American art-house audience will buy some simplistic ideas because they come wrapped in a polished European casing"
ouch!
I suggest the cliche of warm Italian and cold German is not found in the film. Certainly the Italian character is gregarious and engaging but several Germans (Lina's mom, the pregnant assistant chef) appear warm and engaging.
Southern Germans, like southern Americans, tend to be warm and gracious by comparison to their northern counterparts. To comment thus is not to paint all with the same brush but to observe a general characteristic that reveals an underlying truth.
Mario's success in getting Lina to eat is not a product of "smiles, some singing, and some good pasta" but a rather more subtle psychological ploy often used successfully with children "acting out." Mario leaves the pasta bowl and walks away. Nettlebeck shows Lina looking at Mario and Mario completely ignoring her, next scene, Lina is eating.
Although I would agree the ending was a trifle pat, Nettlebeck avoids the penultimate pitfall of healing Martha through love.
Finally, the thought that "true to life... real German women go hunting for Latin lovers on vacation" sounds like someone has internalized one of the cliches he rails against.
With all respect and an up front admission that I know Knipp not, I would suggest a quick glance for glass before tossing any stones.