dimmer reduces the max light .. bad dimmer switch?

Hello, I'm going to be running power to my detached garage which is about 200ft from the main panel. It's a 100 amp panel with plenty of room to install a 50 amp breaker for the garage. I have a few questions that hopefully someone can help me out with. First the wire I think I need is 6/3 romex from the main to the sub in the garage, is this the correct gauge and number of wires needed? I will be installing 1.5 pvc in a 20" trench from foundation to garage. The one item I'm not sure on is this. The wire from the main panel will need to travel about 15' through a block foundation that is filled in with dirt. There is zero room for me to enter this space but I was able to feed a string through so I can pull the wire. This area fully enclosed except for a vent (wire will be run through vent) do I need to run the wire through PVC in this area and connect that PVC to the pipe in the trench? If I do need to connect the pipe this could pose a roadblock. The area where the wire comes out is only big enough to feed the wire and I doubt that I could get an 1.5" pipe through. Thanks in advance for any help!

I know we've dealt with this issue before, so please bear with me if I'm repetitive. Last night a 50 amp tripped. 99% sure it was overloaded (dryer & a/c.) Now it will not reset, even after a long "cooling" period. It has happened before and I have replaced many receptacles and most switches just because they needed to be replaced. This house is extremely old and the service panel looks pretty old. It has no "main" throw at the top like most panels. The 50 amp is listed as 1 of 3 breakers called "Main Disconnects" (stop laughing) and controls several breakers beneath it in the panel. My question is this: If i shut down each of the breakers that this 50 amp controls, shouldn't that mean that the only thing that would cause it to trip (instantaneously btw) would be either a bad breaker or one appliance or outlet (i'm thinking the dryer's 220) was inexplicably wired directly to the 50 breaker and is shorting. All of the other breakers under the 50 are off. If the breaker is bad it looks like I have to call someone, which will lead to an enormous expense, as the service panel will have to be moved as well as the utility's service, as they are both installed in locations they are no longer supposed to be. Panel in a closet, service in an enclosed 3 season room. (really, stop laughing) The house is a teardown in 1 more year, i hate continually investing in it. Thanks so much.