Because you are over 3000 ft (assuming that is the TA where you are), you must be flying the appropriate quadrantal Flight Level once you go IFR (you can do this VFR as well of course). As you point out, you are either IFR or illegal the moment you are not in VMC (i.e. less than 1500 m
(edit to fix - not feet) from cloud.
As a note - you can get close to the cloud as a plain PPL because you will still have your minimum forward visibility (which , along with being in sight of ground, defines if you can legally fly in the met conditions).
There is no 'appropriate quadrantal' when climbing/descending or manoeuvring so this is not an issue in scenario 2.
You then flip the big VFR/IFR switch in your head and realise the new rules you have to follow are fly at quadrantal flight level and be 1000 feet above the highest obstacle within 5 miles (which is pretty likely to be true at 3000 ft) and presto chango you are legally ready to dip into the cloud. This process is equally valid if you are below 3000, except you only have the minimum height rule when you are in the clag.
If you are receiving a RIS and have explicitly told the controller you were VFR, it is probably good form to tell him you are now in IMC.
With regard to flight plans. You don't need to be on a flight plan or a clearance in Class G for either IFR or VFR flight. If you are in controlled airspace, you will already be on a flight plan (or infringing
) and if it was a VFR clearance then you have to remain in VMC or ask for it to be changed.