I think it's important to not get ahead of the evidence. It seems very probable that the "Lombards" carried yDna I-L22. We don't know what other y lineages they carried by the time they reached Italy, although based on Paul the Deacon there were probably several. We know, for example, that the invasion force included Saxons, Pannonians, Heruls, Noricans, Bulgars etc. There are also the prior arriving Goths to consider, who might have carried similar haplogroups.
As for total numbers, they're all over the map, from 100,000 to 500,000 (including women and children). The best discussion of the issue I've seen is in Peter Heather's book: "Empires and Barbarians". His pithy comment is that most of the numbers "aren't worth a damn".
All this talk of population replacement is also premature. To the best of my recollection no academic posits total replacement anywhere, not even in north eastern Italy where there is the most evidence of Lombard settlement (and yDna I1). The estimates for the pre-Lombard population are also all over the map.
Our map here at Eupedia shows an average total number for I1 as 7% in Northern Italy and 4% in Tuscany. There is going to be variation even within northern Italy.
http://www.eupedia.com/europe/european_y-dna_haplogroups.shtml
We also have a thread dedicated to I1 in Italy. It can be found here:
http://www.eupedia.com/forum/threads/28667-Distribution-of-I1-in-Italy-(Boattini-et-al-)
According to Boattini, which however well done used small numbers of samples, it reaches 10.5% in Bologna as a whole, 17.5% in Vicenza, much lower numbers elsewhere. Other studies show around 3-4% in most of Sicily, but 18.75% in Caccamo. And so it goes.
A prior paper discusses Lombard dna in Italy. The area is in the northwest: Piemonte. The paper can be found here:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4312042/
It's discussed in this eupedia thread:
http://www.eupedia.com/forum/thread...hern-Italy-(Piemonte)?highlight=Lombard+mtDna
The authors compared actual ancient Lombard dna to that of communities in the area. Unfortunately, they have so far only published mtDna results. They show a stratification where the members of a Partecipanza show descent from the "Lombards", but the other communities don't show the same kind of relationship.
Just so we have a visual, these are the contracts dating back to the 12th century:
There are a number of them in and around Bologna. This shows the typical land and farm buildings:
Here is the headquarters for the particular one studied:
Here are some of the presidents:
A Consorzio:
They all look like a normal cross section of Emilians to me.