A) Does an immobilzed model get only one attack even if supported with a spear? Seems so: "...rolls only one dice regardless of how many attacks he normally has."

B) Can an immobilized model use magic? It does say he can use Will...

One attack only and a model can't cast spells, Immobilise says a model can't do anything else, casting magic is doing something.

A) Yep, only one attack if transfixed. A spear adds one attack, but transfix drops you to one attack no matter how many you have.

B) A model can only cast in the Move phase when it is his turn to move. Transfixed models can't move, so can't cast. Same with wizards engaged in combat.

Thanks