The filter may be lose if this is not the case the threads around the filter may be "stripped". Try changing the filter if this does not work I would take your vehichle to a certified mechanic.
Possibly. Excess oil increases oil pressure and can put an extra strain on gaskets. This is especially the case when "heavier" oil is used (i.e. 15/40) rather than say 5/30 or 0/30. ...
A plugged PVC system will because it increases pressure in the engine, over filling the oil, weak gaskets, manufacturer's poor design, under or over torqueing bolts, defective or improperly installed...