If one token is missing to use another one instead

You could clone your campaign and email asset and work from two campaigns........In the first campaign your smartlist will have a filter saying FirstName is not empty..............this wont need a new token

In the second campaign have a smartlist filter saying FirstName is empty.....and in your cloned email change the token to Company Name

Easy fix, everyone gets an email with either first name or company name visable!

You can not use tokens inside tokens I'm afraid. So you can't do {{lead.First name:default={{company.Company Name}}}}. And can't make else/if statements either. But what you can do is create a new template and use email scripting. To create it. http://developers.marketo.com/documentation/email-scripting/

I think a better idea is to create a formula field in the admin section of Marketo. Call it Letter Salutation or something along those lines. And start off with IF First Name is not empty, then Hello {{lead.First Name}} ... If Last Name is not empty, then Hello {{lead.Last Name}} ... If Full name is empty, then Greetings.

That way you go ahead and put the comma directly into the field and prevent any confusion with how that will work. Then all you need to do is add 

{{lead.Letter Saluation}} to the top of your emails.

Adam is a wizard. Love it.