If a prospect clicks on a custom redirect after filling out a form, will the...

Yes, but only if the Pardot completion action for the custom redirect is set to change the campaign.

No, a Pardot campaign will always stay the same since it's a first touchpoint.

Yes, the Pardot campaign will change based on the page the custom redirect links to.

No, it's not possible to change a Pardot campaign by clicking on a custom redirect.