To ensure you are updating existing Accounts/Contacts on Lead Conversion
Now, how you determine the acctIdToUse and contactIdToUse may require querying and is application-specific
Note that these three lines are superfluous as Lead.Status is set automatically by the leadConvert....
Object-specific Send Email actions, available only on cases and you can use the case-specific Send Email action in Salesforce Classic, Lightning Experience, and Salesforce mobile app.
Unfortunately, Global Send Email actions are supported only in Lightning Experience.
Check Quick Actions
You will need Advertising Studio’s lead capture feature. This will allow you to create lead ads, and have the fields submitted directly into data extensions.
However, before you send out emails to these new contacts, ensure they are provided with an appropriate subscriber key, as they otherwise will be created with their email address as subscriber key, ...