Lombards is romance speaking, their name while Germanic is Langobards.
So let's say that they only succed in conquer in the Poo Valley, and it's harder hit by the Plague of Justinian and the Gothic Wars is even worse for the rural population, and the Lombards move in before the population have regained it former size, set up a lot of Langobard freehold in the empty country side, but the East Roman bribe them to keep them out of the rest of Italy.
There are some non-Tyrolean German dialects spoken by tiny communities in Italy that are commonly alleged to be Lombard/Langobard in origin, such as Mòcheno and Cimbrian, but I'm not sure how sustainable these allegations are.Lombards is romance speaking, their name while Germanic is Langobards.
To make their language survive, you need to make them less succesfull, the problem is that they only mad made up around 5-10% of their territoriums population.
If the Lombards remain Arians rather than converting to Catholicism, you could have a "shield of faith" type situation where they could preserve their language.
While I do agree it's quite a shitty place, is not called the Poo Valley, is the Po Plain or Pianura Padana.
And for Lagobard to survive as an etnicity it have to populate a far less extent and populated place, making a stronghold somewhere peripheric, Trentine or Friuli will be ideal, so will Savoie, Liguria or Vallè d'Aote...