Pre-populate external registration forms (Gravity Forms, Eventbrite, Absorb LMS) from existing Salesforce Contact data

We're a nonprofit on NPSP. Our registrations come in through three external tools:

• Gravity Forms (WordPress site)
• Eventbrite
• Absorb LMS (new training platform, launching January)

Today, Zapier pushes submissions into Salesforce. It matches Contacts across multiple email fields (Work, Alternate, Personal) and flags name/email mismatches for review.

Use case: a person who's already in Salesforce opens one of these registration forms. We'd like the form to pre-fill with their Salesforce data (name, organization, title, phone, address) so they can review and correct it before submitting. The corrections would then flow back to Salesforce.

Questions:

1. What approaches have you used to pre-fill an external form from a Salesforce record? For example, personalized links with URL parameters, a lookup-by-email step, or a Salesforce-native form tool.
2. How do you identify the person safely? We want to avoid showing someone's Salesforce data to anyone who just types in their email.
3. Has anyone done this with Gravity Forms specifically?
4. For tools that can't be pre-filled, like Eventbrite or Absorb, is the usual answer our controlled form in front of them?

Thanks in advance!