Snapedom

As Terri said, Severus did *not* know Nagini was protected until he saw her in her bubble in the Shack. Severus was at the school, Voldemort was touring the country for his Horcruxes. The only thing Severus knew was that Voldemort expected Harry to go to Ravenclaw tower and that eventually Voldemort would arrive later because one of the Carrows had summoned him through the Dark Mark. But he did not know if this was going to be their final show-down or just yet another encounter as they have had over the years. He had no way of knowing it was the time to deliver his message.

Regarding the Patronus - it isn't the fact that Severus could cast a Patronus that mattered, nor the aesthetics of the Patronus, but the fact that Harry would realize the moment he saw Severus casting it (whether in life or in memory) that it was Severus who had brought him the sword in the forest. Harry knows why the sword was delivered to him (even if Severus didn't) - so he could destroy Horcruxes with it. And he knew through Albus' will that it was part of the Great Plan that he have it. So whoever bothered to make the effort and track him down in a forest (where he was so hidden that even Ron only found him with the help of Albus' Deluminator) to bring him the sword that Albus wanted him to have, the sword he needed for his mission, had to be on Albus' side, and therefore on his.

Obviously Harry wouldn't have trusted Severus' Patronus earlier - which is why Severus doesn't let the Patronus speak. If the Patronus had said anything, however well meant, in Severus' voice the sword would have been left lying under the lake.