You should definitely be able to include information from the Owning user in the workflow, and you definitely should not need (and do not want) to copy the "outside company" field to lots of lead or contact records (and by the way, mapping only works at the point you create a record from inside the context of the parent, so this will never work retro0actively against existing records).
I am not certain if the email template includes fields from the user who creates the email rather than the owner, (so in this case the workflow owner is the one who creates the email), but that makes sense if it is the case. So you may have to build and send the email in the workflow rather than using an email template.