Clarification of a proof as written in Margaris's book

Question

In Angelo Margaris's book First Order Mathematical Logic it is written (see below),

Then,

Questions

1. In the above proof I don't understand what am I supposed to do at the second step. Can anyone explain that to me?

2. How from "$P$ admits $t$ for $v$" it follows that "$\sim P$ admits $t$ for $v$" (because otherwise you can't use Axiom 5 which says that $\forall v Q\to Q(t/v)$ provides $Q$ admits $t$ for $v$)?