As long as they use the email address you have stored in SFDC, your plan should work fine. If they're getting the landing page from an email that you're sending out, that email will pre-fill in the form (if I remember correctly). If they've already filled out any Marketo form and the form on this particular landing pages has pre-filling enabled, I don't think you'd run into any issues.
The person in SFDC should be tied to an account, so as long as you don't have duplicates of that person in Marketo & SFDC, once they fill out your form, the post-form submit landing page token idea should work.