2 tanong about Snape & Voldermort

1) That was a risk he had to take. Besides, he must have been trained sa pamamagitan ng Dumbledore, the best.

2) I think he did, in the last minutes. But he had probably thought that Voldemort was cleverer than that and would have understood what Ollivander had told him. But, alas, not, as Voldemort doesn't pay attention to tiny details that matter most.
Besides, Severus had taken an Unbreakable Vow, whose one clause stated that he had to protect Draco from harm. Voldemort would have understood that Draco was the true Master of the Elder Wand and would have gone after him. Seveurs killed two birds with one stone : a) protect Draco ; b) let Voldemort think he was now the Master and be too sure of himself, while he was not really.

1) I think Severus had delved into Legilimency and Occlumency when he was still a student at Hogwarts. Regardless of who taught him, he surpassed them; Severus Snape is regarded as the most powerful Occlumens in the world, as well as the segundo most powerful Legilimens. Dumbledore is very close behind our Potions Master. Lucius, Narcissa, Bellatrix and Draco are also quite skilled, though none of them are near the level of Voldemort, Severus, or Dumbledore. Kingsley Shacklebolt is also quite skilled at both. Anyway, as LN said, it was a calculated risk that Severus had to take. And it motivated him to become as powerful as he did. Equally, he would have to be a skilled Occlumens to shield parts of his mind from Dumbledore, who was powerful enough at Legilimency to skim thoughts from the minds of those around him. (This happened most frequently with Harry, and I think we all know why.)

2) In the book, oh yes. Severus suspected, because when he sagot the Dark Lord's summons, he does so with wand in hand. That was one of the reasons why, in the book, he keeps asking Voldemort to let him "find the boy", because he knew Voldemort was done with him. In the movie, obviously it's depicted differently. I especially enjoyed the placating way Severus speaks to Voldemort, like one would to calm a fussy child having a tantrum.