Securing front engine on articulates...

How I deal with the front engine depends entirely on what I'm doing to the locomotive. Generally the locomotive is going to be sitting on its wheels, or on its side if I'm doing traction tires. If I have to have the unit in my hands for some reason I may remove the front engine entirely, depending on how hard it is to do so. Sometimes there's a single wiring connector and you can just pop the front engine off. 

For reassembly, the key is to be sure you get the connection between the boiler and the front engine right. Some Lionel engines have kind of a keyway with a prong that has to be inserted correctly into a slot, and kept in place while you attach the boiler to the rear engine. I don't have a set procedure for reassembly; I play it by ear according to how the specific locomotive goes together. 

If I have one upside down, it's in the foam cradle.  I normally set the boiler on right side up, I just find it easier.  Then I flop the whole assembly into the cradle and put the screws in.