When Spike is trying to communicate with Fred she is in the shower and he is standing outside the shower stall. He would not have been able to write on the glass since the shower steam would have fogged up the inside of the shower stall glass and not the outside where Spike is.
As Spike is non-corporal, he should not be able to sit on the sofa next to Angel.
Spike is supposed to be non-corporeal, however he hits ground whenever he falls to the basement. He later tells the Reaper he has figured out that reality bends to desire, allowing him to fight, but it is unlikely that he desired to hit the ground while falling.
When talking about the plan to help Spike, Fred says they'll need a massive amount of dark energy - "the equivalent of nucular evil." Fred wouldn't mispronounce nuclear.