Is it possible to create a 'send by email' button/field without using lifecycle designer?

Sure, you can add a button in Acrobat and then set its MouseUp action to submit the form by email.

As the target URL enter something like this:

mailto:me@server.com

(use your own email address, of course)

Flattening the file is only possible if the use has Acrobat. Assuming they use the free Reader the best that can be done is either to digitally sign the file, which doesn't flatten it but it does prevent the fields from being edited, or to set the fields as read-only using a simple script.