1) Should BA be providing (or at least paying for) overnight accommodation in BKK ?
2) Presumably EC261 compensation will be due ?
3) Presumably no further compensation for the horror of travelling a B777 compared to the A380 she was originally booked on :-)
4) Out of curiosity, since it is too late to fix this now, the QF1 from SIN to LHR leaves about the same time as the BA12. This wasn't offered as an option but would the agent have been able to re-book on this flight. I can see that there are seats available.
1) Yes, in fact overall it's best to book the hotel yourself, there isn't much benefit in BA doing so. So just book the Novotel in the airport, or somewhere on the Skytrain. The guideline is £200 per room per night.
2) Presumably yes, unless the damage was caused by forces external to BA's control (e.g. another aircraft pranging it). See the main EC261 thread in the Dashboard.
3) Nope
4) Which is why it's always good to research before calling, yes that would have been possible but the preference is for BA metal. There is usually good advice to be had in this forum, or use ExpertFlyer or similar tools.