I would be interested to find out what area you were searching so I can check our data quality in that area.

There are many many web sites offering access to foreclosure and related data.

My objective is to identify great potential 2-4 multifamily properties, what questions would you ask Mashvisor if they tell that they do not list lots of 2-4 MF deals, how does affect their tool in finding and analyzing deals through out state of CT, and how can you verify that their data is correct when analyzing multifamily properties

Obviously this is just a guess and not actual data but I would tend to agree that the majority do violate laws, especially today since many of the laws have been added.

My first thoughts were as follows - 1) to do a 10k a month letter/postcard campaign to targeted data sets that I craft personally using buying criteria (geographic location, equity %, price range, etc.) that I supply personally.

Where do you tend to source this data?

This is capitalism making use of data. 

Where do you guys turn for high quality data?

Are these "foreclosure listings" from your data base REOs that are currently for sale?

MS Excel would work for tracking the data and can update a schedule E template for you for each property.