1. Most likely your Person Score is the same field as Lead Score. There's been UI updates to try get terminology the same, but as you've found out, it hasn't been done very well.
2. It's most likely because Person Score is the same field as Lead Score
3. Don't you already have fields created "mkto2__Lead_Score__c"?
As Jay mentioned, Lead is now Person. Newer instances would have the "Person" description available.
If you do not already have the Person Score/Lead Score value in SFDC, you would need to create a custom field in SFDC with
Instructions: Adding Custom Marketo Fields to Salesforce
Field Label: Person Score
Field Name: mkto71_Lead_Score
Data Type: Number
Field Attributes: Length 10 , Decimal Places 0
Hope that helps