I would have to disagree with your opinion on level actions they where a revolutionary leap in technology and in certain battles made a huge difference.Case in point the Battle of Little Big Horn the US Army mainly had carbine Sharps rifles a single shot breech loader.The Sioux who also used a good number of firearms but they really liked Winchester and Henry lever actions which by the the US Army never adopted a lever action but it was not because the design was unsound they thought troops would expend ammo to quickly.
Perhaps the particular rifle you had was the problem I have not really heard of the issues you mention with most level actions unless of course something was wrong with them.
Lever actions where an important step towards fully self loading rifles.
The more impressive thing about the Sten gun is that is pretty much made up of scrap metal and it could be field stripped in seconds which allowed insurgents to easily hide them.Sten guns only cost about $10.00 a pop to produce which is pretty cheap.I have not heard before that the Sten could use MP-40 mags before.
"I read something somewhere that you should never trust your life with a weapon that is not your own. You don't know how the enemy kept their weapons and the last time they were cleaned...not to mention you might not be familiar with that type of weapon."
Not true by any means.You can learn to operate any firearm they really are not that complex because the person who may use it could be pretty stupid.You do not need to understand basic math or read and you could be taught in a few hours how to operate and maintain any small arm.Most soldiers at some point are trained on several weapons including enemy ones once they have done that they know enough to be able to operate and maintain any firearm they encounter.Any elite soldier is expected to be able to use any weapon be it an M-4 or some farmers old bolt action rifle.