"To" doesn't serve as a pronoun here, it takes the role of the subject - for which the English equivalent is either this or these. Therefore it's always 'to' no matter whether we're talking about masculine, feminine, neuter or plural. And such basic constructions "This is X" and "These are Y" simply look like this every time.
To jest -nominative singular-
To są -nominative plural-
Someone suggested elsewhere that when the instrumental case is used with być, it is like saying someone or something is being in the manner of something else. It is rather a redefinition than a mere definition.
On the other hand, we use To jest to introduce something for the first time. It is not a redefinition, so we can make sense that the instrumental case does not apply.
No. First of all, this is an "This is Y" type of sentences, so there's no he/she/it here. Secondly, if I were really to use a similar construction (let's say that I'm talking about Hektor, and my interlocutor thinks that Hektor is a person, but in fact Hektor was my friend's dog), I would use "on" or "ona" depending on the dog's gender, and not neuter "ono".