Let’s begin with the second question.

In fact we have an infinite collection of symbols.

We can use one symbol with subscripts on it.

For the first question, I have never seen existential instantiation put that way.

I can only guess what the textbook means.

Let me give you an example.

There is a brown object.

There is a dog.

Therefore, there is a brown dog.

Clearly that is not a valid argument.

But without the rule for existential instantiation we could us the same constant twice and arrive at that conclusion as valid.