Interesting DFC thanks for your post; my thoughts are as follows:
1) An FI can no longer be 'freelance' in the sense that he used to be. In the old days he could just work from home as it were. Now he must work through a registered flight training organisation but there is nothing in the ANO or in any rule which prescribes what his tax situation must be. Consequently there is no problem with such work being done by self employed instructors, registered with and working through, such an organisation. In that event there is no question of the minimum wage.
2) Trial Flights by PPL FI is a very interesting question and you raise a good point. It may well be that some such flights at some places could be illegal but I do not think that it is wise to assert that all are. It depends in any given instance on the exact circumstances and I can see that some arrangements would be better than others.