Will an aircraft always crash on take-off if de-icing is not performed correctly?
The claim that de-icing must have been done perfectly in Brussels or the plane would have crashed on take-off is patent nonsense. Ice on the aircraft degrades performance. A little bit of ice degrades performance a little bit; a lot of ice degrades performance a lot. It's perfectly possible that they took off from Brussels with slightly degraded performance, realised that there was still some ice on the plane and decided they needed to get rid of it.

There's no reason to suspect a cover-up. If there was even a hint of terrorism, the plane would have diverted to Stansted, not Heathrow. If the plane had some mechanical problem, they'd have made a vague statement about the plane having mechanical problems, landed, fixed the problem (which requires maintenance technicians, not a de-icing crew), and then said something much less vague to reassure the 227 passengers that the plane was safe. If there was a problem with the passengers, luggage or cargo, the plane would have gone to a terminal. If a vice-president of the company had been served macadamia nuts in the wrong way, they'd have dealt with that in Brussels.