The problems with interpretation of the law is it is just that. With no test cases to quote your are wholly left to the discretion of lawyers judges etc and we all know how they can disagree amongst themselves, that is one of the reasons we we have several levels of appeal in this country.
The reasons that instructional flying for ratings and licences must start and end at licensed airfield is because of the increased potential for accident, that is my deduction. However it would be my contention in any court that if the instructor was the sole handling pilot such a requirement although illegal could be overlooked in certain situations, again that is my opinion.
In regard to your ferry question any comment on here is purely speculative for the reasons given above. Take for instance a flight back to a licensed aerodrome where the fireman on duty goes sick and you land back illegally(you should have diverted of course) that is a much different scenario from when the intent before flight was to illegally conduct a training flight between two unlicensed airfields.
Much time of these forums is spent speculating about unenforceable laws. To put it in perspective it is still illegal in this country not to report to the lord of the manor for regular long bow training.
It is important to operate within the spirit of the law and apply common sense to why the law is in place.
I give you a classic example from many years ago when log books could be checked by the CAA while you waited.
Flt Lt Jones was ex RAF transport command(26 years) and had just been demobbed and had brought his log books to the CAA to have them checked for the issue of a CPL.
He waited a few hours and eventually the clerk came back and said. We have checked your log books and we can see no evidence of the required triangular cross country.
Ah but Jones said i have flown around the world 9 times.
Yes said the clerk but that is not a triangular cross country!