Sensor ontop of the transmission. There should not be a sensor on the top of the transmission. It maybe a vent hose attached to a fitting on the top of the trans.
But as for your grinding I would have the torque converter inspected. The grinding maybe the bolts hiting the inspection plate. Which you hear at idle but due to engine noise you can't here anymore. But best to have it looked at.
The bellhousing is aluminum. There for it must have been tig welded backtogether. I highly doubt it is warped. But if it needs to be replaced you need to have the whole transmission housing replaced. The bellhousing is part of the transmission case. Unless it is the bell housing section bolted to the engine. Which in that case that part can be replaced.
Now As for your crank will not start. I believe that is not related to your transmission at all. Most likely that is a seperate issue. In which I would first start by scanning for trouble codes. Checking the battery voltage, and go from there.