Deloitte Internship guaranteed return offer

Yes, if you intern at Deloitte, unless you're really bad, like just don't show up to anything, and put no effort into your tasks, then you should have a FT offer at the end of your internship. You have ~2-3 weeks to make a decision to accept your return offer once you receive it

It is possible to re-recruit for a different branch of Deloitte for undergrads. You're taking a risk though, because if you're not successful, then you'll find yourself with no Deloitte offer. For example, I know some audit or advisory interns who at attempt to re-recruit for Deloitte Consulting. Typically, I advise, unless your tax, to just accept your offer if you want to do Big4. Down the road, no one is going to really care if you were advisory, what matters more is the experience you gain. At the end of the day, you'll have 'Deloitte' on your resume. In addition, you can always either network your way into a consulting engagement (not necessary at all) or you could even switch to another Big4 down the road to their consulting arm. Once you're in Big 4, it's fairly easy to switch between firms.