What a very sad situation, and even though I did not get to use your services, I sincerely hope that you manage to fall back on your feet very promptly and resume a healthy and prosperous business.

Very sadly, a third and fourth "take home" insights from your experience are:

1) there have been many debates on whether hidden city ticketing issues are enforceable and while some of us have insisted that people should be cautious and have read or heard a number of bad experiences, many have argued that they had never heard a case of BA punishing offences themselves. Sadly, they will not be able to say that any more;

2) there have been rumours that BA was trying to enforce those rules more strictly and again, this cannot be doubted any more.

It seems quite tragic that from what you describe, BA seem to want to make "an example" of your case. I genuinely hope that they will consider that you are hardly in a position to control the behaviour of your customers. I suspect, however, that this may proceed from BA actually explicitly looking into such "suspicious" constructions as overnight stops and LHR/LGW connections that could be avoided as I suspect that this is the only indication that they could use to try and blame you for customers misbehaving.

Once again, though, beyond those "rational" considerations, the more important human answer is that I really hope that things go back on track for you and that this soon becomes a mere (very) bad memory.