Flying under IMC

Hi,
i'm asking myself to make an Instrument Rating or not.
There are some questions to IFR-Pilots :

... when flying under a flight plan (IFR) can i fly routes at my choice ?
e.g. take-off under IMC, enroute 10 min under IFR (clouds), 30 min VFR on
top, descending to my destination under IMC (through cloúds), landing
at destination under VMC (below the lower layer of clouds)

... when reaching IMC after 30 min. VMC ("round trip under VFR") do i have
to file a flight plan (IFR) or am i able to fly only by reference to the
instruments through an e.g. 300 ft Layer of clouds and after that, when
reaching "On Top" fly again under VFR ?

Peter