If the users are using Reader then you can't actually flatten the file, but you can set the fields to be read-only using a script.
And no conversion is necessary to submit the file by email. A PDF (with the proper rights, applied in Acrobat beforehand) can be submitted via email by the user, directly from Reader.
The Signauture field has a built-in option to make fields read-only once its signed. If you want to also email the file after the signing you will need to use a script to do it, though.
The users will have writer(9 or 10)
If you mean Acrobat, then it is possible to flatten the field, but why would you want to do it, actually?
The signature guarantees that the file has not been changed since it was flattened, while a flattened file can still be manipulated and you will be none the wiser (since the signature is meaningless at that point). If you want a guarantee that the file hasn't been tempered with after it was signed, the signature itself is that guarantee.