First of all, the house show rumor is just that, a rumor. WWE has to advertise Batista as the world heavyweight champion because WM hasn't happened yet. Batista is currently the World Heavyweight Champion, so they have to advertise for that, otherwise, should they choose to let Taker win, it would spoil the outcome of Wrestlemania.
Secondly, I can say it's possible for Undertaker to lose at Wrestlemania, but not likely. I mean, I seriously doubt Vince is going to let both Champions lose their titles at Wrestlemania. Although, I think it would be the greatest thing Vince has done in recent memory. I think Taker will win at Wrestlemania by a pinfall.
Thirdly, the whole thing about J.R. announcing the Cena/HBK match last is because it was RAw. They are Raw superstars. I bet on Smackdown, JBL and Michael Cole will announce the Undertaker/Batista match last. Besides, I remember reading on this site not long ago, Jim Ross stating that the last match on the card will be Undertaker and Batista.
Lastly, Vince McMahon has made stupid decisions here lately. I have not been impressed with him over the past few years. I mean seriously, he caused Edge to job to Cena in Edge's on hometown!!! I bet if they were in Boston, Cena would have won. Vince choosing Batista to win would be an outrage and I for one, would quit watching Smackdown and possibly even Raw. I am giving WWE the benefit of the doubt by ordering Wrestlemania. If it sucks anything like last year's did, I will no longer be a WWe fan.