They would not have Cena beat Taker at Mania, not as long as they want to keep Cena a face. The huge '05-'06 backlash against him would be back immediately, if not stronger.
Not if it's done correctly.
Make it a respect thing. Everyone knows Undertaker is about to retire. They can have it similar to last years, where Undertaker puts out an open challenge, and Cena responds to it, saying how facing Undertaker at Wrestlemania is the biggest of the big, blah blah blah, both men put in their very best, blah blah, and in the main event, Cena prevails in a very back-and-forth match, where Undertaker raises Cena's hand in victory, Cena claps him and stuff, yada yada.