Has anyone had good or bad results from trying out credit report and score businesses that aim to improve credit and help in the journey to gaining a pre approval for a mortgage?

The LTV will be based off of credit score, property type, number of units and loan amount.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

The good news is he had a very solid explanation for why this was all happening which is what got him over the hurdle.As you mentioned this is lender specific but 620 is the minimum FICO score required. 

We spoke with a credit union who said that would be possible; however the loan was not approved due to both of us being NY residents.

I had multiple FIVRR folks do work but a lot of them will create back links on sites with high toxicity scores so it actually ends up hurting you.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Varies a lot with credit score and experience.

Unfortunately, investors typically only look at offense as that is where the points are typically scored and how well you mitigate risk is tougher to grade - until you lose it all.