@Mary
1. Yes, send all emails from within Marketo and do not forward anything sent directly to one of your employees. The best practice is to add a new lead and send them the email. Emails sent from Marketo track the recipients behavior. If the recipient forwards that email around it would be as if all of that activity was performed by the recipient, including reaching a page with a form that is pre-filled.
2. If you don't like the above, you can also choose to disable link tracking in Marketo emails by adding the .mktNoTrack class to links that you wish not to be tracked. See
this article for more details.
3. By default, any email sent from Marketo is tailored for the recipient and tracked accordingly. To get a prospect into Marketo, you should either share the actual URL of the landing page in a separate email so that they reach the page as an anonymous visitor, add them in Marketo and send the email to them directly (#1 above), or send them an email with tracking disabled (#2 above).