To answer your second question first, no, it is necessary to have Word to use a Word form. If you aren't sure that all recipients will have Word, a better choice would be a PDF form, but you must have Adobe Acrobat to create that.
I don't know why you would instruct users to close the document and then say yes when asked if they want to save changes. That sounds risky to me compared to just saving and then closing. But if the document was opened from an attachment, it won't be permanently saved in any case (this is another issue). It might, however, be possible to email the completed, "saved" form back from its temporary save location.