Solenoid has nothing to do with fuel to the carb. It has a pin that blocks the main jet when not "energized", preventing after fire. Fuel is leaking past the needle/seat in the carb, overfilling the bowl and running into the intake and then the cylinders. It then seeps past the rings and into the crankcase. You might have a speck of crud preventing the needle from shutting off fuel flow. This often happens when changing a fuel line or filter. You basically have to remove the bowl and float & needle. (Clamp the fuel line first) Unclamp the fuel line and let gas "flush" things for a couple seconds. Cross fingers and reassemble.

Marty dumping fuel through the venturi on CV twin is typically the float valve not seating. The floats are mostly not adjustable, but should be checked for saturation. You will need to know that the vent for the bowl is open. Other than that , some had an accelerator pump on the side- 3 small screws in the cover which will cause a rich condition. There are several variations of carbs on these and you will need to toss the model and spec on the googler and look up some parts. Click on the carburetor in the diagram to see the service parts used, or replace the carb complete.

Could be. If the oil is not contaminated w/ fuel and it is pumping oil mist from the breather hose into the air box- it would be a likely scenario. So start with an analysis of what the oil looks like on the stick and go from there. IE hard to read, reads high, feels thin, smells of fuel etc. would be the signs of contaminated oil.

If all seems physically OK in the seal area, my guess is that you are building crankcase pressure. Does that seem to be the case? That is if it isn't tearing the seal from a mar on the crank. Does the seal look stressed from what you can tell after removal?Is it being installed at the proper depth and not partially over an oil passage? I have not had that experience nor do I really work on many Command horizontals. I've had issues on other horizontals that had other external leaks and would actually blow oil though. As an example, a Briggs with a missing dipstick seal. So if you are building pressure, a likely failure would be a head gasket or a breather issue. I do remember though that the horizontal's breather reed is under the breather cover. I would assume that there is a drain hole near the bottom of the breather chamber too. So, depending on engine condition that gives you some things to consider. You may want to even try a new oil filter (OEM) and oil (10W30) for starters if you haven't so far.