Note: The literal translation for “share” is 分享 or 共享, not 转发.

You are right. This is a sentence which leaves the plurality undefined, either the speaker thinks it is not important for the listener (no difference for him that the message is forwarded to 1 friend or to many friends), or the speaker just does not want to specify it.

There are certainly ways to express different situations in Chinese if the speaker wants to.
e.g.
If you don't need this book, give it to a friend./如果你不需要这本书，送给朋友吧。
Here only 1 friend can get the book (as there is only 1 book), but which friend gets it is probably not defined yet.

I would like to give this book to a friend/我想把这本书送给一个朋友。
Here also only 1 friend can get the book. We can understand somehow from the context that the friend is actually already defined, although the listener does not know his/her exact identity.

This is in the context of something online. So you 'forward' an email or text message or something like that. The meaning is you are passing along a message you received from somebody else.

There is actually a neat rule in English that allows nouns to be turned into verbs and vice versa.

There is no plural in Chinese. 朋友 here can already mean a friend or friends. It is not necessary to attach 们 except when a sense of unity and group is needed.

Correct.

把 introduces a direct object. Think of this as "take (object) and (verb) it"

The meaning of 把 is similar to render.
(A) 我吃了那个苹果 I have eaten that apple.
(B) 我把那个苹果吃了 I have rendered that apple eaten (by me).
Both sentence means more or less the same thing

"这个" means "this".