If Undertaker hadn't missed Wrestlemania X and Wrestlemania XVI because of injuries he sustained those times do you think Undertaker's Undefeated Streak would 19-0.

Yes, I think it is very likely.

How do both The Big Show as well as Kane and The Undertaker get around? Since all three are very imposing and incredibly large men what kind of transportation do they normally use to get around from one place to another?

Big Show actually travels in a large tour bus when on the road.  Kane and Undertaker, obviously, are going to rent larger cars and travel in business or first class on flights. 

When Shawn Michaels required back surgery during 1998 after WM14. Was the initial diagnosis that he wouldn't return and they'd have to use him in a different role, IE Commissioner, or was it more of 'Shawn's gonna be out a while but will be back in months/a year' thing?

Shawn was giving the impression that his career was over.  Many were skeptical, but WWE went forward with the idea that he wouldn't be wrestling again for quite some time, if ever.  

Should Demolition be in the Hall Of Fame?

I absolutely feel Demolition belongs in the WWE Hall Of Fame.  When you consider that they were written off as Road Warrior ripoffs, but managed to overcome that stigma to become a top team (until the real LOD showed up, and Demolition got de-pushed), I think their run makes them worthy, certainly as much as Koko B. Ware.