Ya, I've also heard of that method. Either way, warming the can up a little works wonders in getting the gas out.
To answer your 1st question, the R134-a carries particles of these materials with it as it flows, in gas or liquid form. Just like in your a/c system, the oil in the system is made to be carried with the refrigerant used, so that the refrigerant will deliver it through to the various components (especially the compressor) as it moves through the system. This is why you can't use R-12's mineral oil in a R134a system. R134a is not compatible with mineral oil and will not carry the oil through the system very well at all.
By the way, I think the stop leaks and such contained in those cans are frowned upon by a/c technicians. Im not one of them, so I don't know for sure.. but it's one of the reasons they are nicknamed "death kits". They are known to plug up expansion valves and cause issues with gumming up, etc.
To your last point.. I think you are probably right to a certain extent. At idle the "suction" is probably slow enough that any liquid being added has a chance to convert to a gas before entering the compressor. The suction side is cold, though, not hot.
I'm also guilty of having tilted the can when charging in a hurry. It really is playing with fire though from what I've read. When I did it I would more swirl the can in the upright position and briefly turn it upside down, because I knew I was risking it.
