If it was an ERS issue it should be obvious as there's an ERS status light on the cars; it would be all over the media by now (as a confirmed fact, not a rumour) if the McLaren had a big red light on it after the accident. All indications are it's an impact (either caused by driver error, wind or car issues, who knows) that, whilst maybe at relatively low speed, was violent enough to give Alonso concussion. The speed is by no means the only factor that determines how bad the injury is, Alonso probably just hit the wall at an awkward angle or something.
Also, the fact that he's in intensive care just means the hospital are monitoring him closely to ensure the concussion doesn't cause complications as a precaution. All the information we have suggests he'll be leaving hospital within the next couple of days once any risk from the concussion subsides. There's all sorts of speculation in a lot of places and it's unnecessary and has got wildly out of control, in my opinion.
