The Salesforce connector is a feature that allows you to integrate Marketing Cloud Account Engagement with Salesforce, a customer relationship management (CRM) platform. With this connector, you can sync data, activities, and campaigns between Marketing Cloud Account Engagement and Salesforce. You can only have one Salesforce connector at a time, which means that you can only tie one Salesforce account to one Marketing Cloud Account Engagement account. You cannot have multiple Salesforce accounts linked to one Marketing Cloud Account Engagement account, regardless of the number of accounts, the edition of Marketing Cloud Account Engagement, or the support from Marketing Cloud Account Engagement. Reference: Salesforce Connector, Connect Marketing Cloud Account Engagement and Salesforce

The three things that will affect the prospect records based on these activities are:
Bob's prospect record will show him clicking on the link and filling out the form. This is because when an unknown visitor fills out a form, they become a known prospect and their previous activities are tracked and associated with their prospect record1.
Kate's prospect record will show her clicking on the link and filling out the form. This is because Marketing Cloud Account Engagement tracks the activities of known prospects who interact with your marketing assets, such as custom redirects and forms2.
Kate's prospect record will have a tag of "clicked" applied to it. This is because the custom redirect has a completion action to add a tag of "clicked" to all prospects that click it, and completion actions are actions that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity

The best way to configure a thank you email to be sent after the form on the "Request a Demo" landing page is submitted is to configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted. An autoresponder email is an email that is automatically sent to a prospect after they complete a desired activity, such as submitting a form or clicking a custom link. You can use autoresponder emails to thank your prospects, confirm their actions, or provide them with additional information or content. A completion action is an action that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity, such as submitting a form or clicking a custom link. You can use completion actions to perform actions on the prospects who complete the activity, such as adding them to a list, assigning them to a user, or changing their field values. To configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted, you need to go to Marketing > Forms > Forms > Request a Demo > Completion Actions > Add New Action > Send Autoresponder Email, and select the email that you want to send.

In Pardot, new custom fields for prospects are created in the Prospect Fields page under the admin settings. Once a field is created here, it can then be added to forms as needed. This process ensures that all data captured through forms is aligned with the prospect data structure already established in the system, allowing for consistent data management and integration with Salesforce CRM fields if required. This separation of field creation and form editing ensures organized management of prospect data attributes.