Some inspectors may eyeball the work performed and feel it is sufficient to sign off and yet others will ask you to open up the walls to ensure everything was done to code.

With that said, the inspector had no issue with there being a 3rd unit.

I have even seen twice city inspectors walk buildings for other issues that have illegal basement units and as long as unit looks like a legal unit (normal ceiling height and two egresses) they didn't even say anything. 

Inspectors will check for leaks, sagging, or missing shingles, particularly in older homes.

Since you're talking about a leak...Did they have a plumber inspect or a roofer inspect?

The inspector might have screwed up which could put their insurance at risk.

About 5 days before closing, I found out there are POS violations; I was told by my realtor the old report expired and the inspector went out on 7/1.

(is there any reviews or history on his business online)The inspector that said 70+ is probably one of those everything needs to be replaced don't risk type, however will probably be the more expensive guy.

Than we need on-staff inspectors for the move-in/move-out process and being boot's on, 2-4 persons there.

I am an InterNACHI Certified Professional Inspector, working with any home buyer or seller.