With dual licences and different rules what you can log (biggest difference: both logging PIC when flying with an instructor when you are rated on an aircraft) you can't keep a single logbook that nicely adds up in both FAA and JAR terms.
You can either keep two logbooks or you can log, say, according to JAR and when you ever have to add up total hours for FAA further licences you can include/exclude hours according to FAA rules. That is what I do - I have one JAR logbook - although I have a standalone FAA PPL/IR as well as a JAR CPL/IR.
Now to your specific questions
- Log Day as Day
- Log Night as Night (
AFAIK all logbooks log night separately)
- Log time with "sole reference to instruments" ("instrument flight time" in FAA speak). This is the important number for higher licences both in FAA and JAA land.
The above is 100% clear cut - it should be clear at all times if you are flying at day or night, and whether you fly by sole reference to instruments or not.
Now to IFR flight.
1) Don't log night flight in the UK as "IFR flight". Just don't.
2) Log airways flight or anly other flight operated primarily under an IFR ATC clearance as IFR
3) Grey area: IFR flight OCAS. I would personally only log such a flight as IFR if
- I prepared and conducted it with the expectation to fly the majority of it in IMC,
- They are an off-airways extension of an airways flight, for example when going to Newquay and the airway ends somewhere over Devon...
If I pop up through a layer to go on top on a beautiful day and pop down later again I don't consider the overall flight as IFR flight, notwithstanding the 5 minutes of IFR taking place, and just log the 5 minutes as sole ref.
Of course you can consider yourself flying IAW IFR at night and at day in brilliant weather, but whom are you cheating with logging that?