Why je rather than jsou (since "them" is not a single item)?
Mnoho and other adverbs of quantity use singular and are followed by a noun in genitive.
Why is "jich je mhoho" not accepted?
Because it sounds rather unnatural (at least to me).
I am Polish and keep comparing it to my native language. In Polish it would be: "Jest ich wielu!" and "Ich jest wielu!". Both sounds natural. Maybe with more emphasis on "them" in second sentence.
also "je mnoho jich" is unacceptable. so in this sentence there is only one word order it seems
Jich je mnoho! is also accepted.
Could you say 'to je jich mnoho'?
No, there is no place for TO here.