You seem more worried about this car than pretty much anyone on any forum, except maybe Ford owners, They have a LOT to worry about.My car is a 2006 Civic EX Sedan as well and my s*** is falling apart as well. I need to replace both front wheel bearings in a couple of days. Mine has 164k on the odometer. After this, i'm not dumping anymore money onto the car other than tires and 4 struts.
You did say that you got it 4 months ago. It's possible that the last person screwed you over and knew the block was going to crack so they sold it asap.
Maybe it is time to move on while the engine is still good. It may never even crack.
About buying a used car - one often thinks the previous owner "must have known" or did something to temporarily mask a problem but there isn't any way to really know.