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