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Thank you very much, that was really helpful.
(4) What about deriving a - for example Java - class from a class under (a) MPL (b) LGPL Would the derived class be allowed to have a proprietary license?
I don't know the Java language, so I don't know what "derive" means in that context. So I'll go for: Maybe :-)
There might be some helpful info here: http://www.gnu.org/licenses/lgpl-java.html
Now, from what you said, also this issue seems pretty clear. A derived class, that is inheriting attributes and behaviour from another class is a derivative work. However, I think, since the derived class normally is in a separate file only the LGPL requires it to stay LGPL. The MPL should allow another license for that file/class, even though it is strongly connected to - and definitely does not make sense in any way without - its super/father/base class.
Regards, gk