A total score of 29 UMD is enough to cast a spell without any chance to fail.
There are two checks, one to be able to use the scroll and one to be able to cast the spell successfully.
To be able to use a scroll you must have a total UMD score of :
Lvl 1(DC13), Lvl 2(DC15), Lvl 3(DC17), Lvl 4(DC19), Lvl 5(DC21), Lvl 6(DC23), Lvl 7(DC25), Lvl 8(DC27), Lvl 9(DC29)
So at least 21 UMD for your level 5 spell, no dice roll here.
And to cast something successfully you must pass the second check :
25 + spell level vs UMD score + d20
So in your case, with a 5th level spell and assuming you roll a 1 it becomes 25+5 versus UMD score +1. So to have 100% success you must have at least 29 UMD.
As for the 2 other questions I never checked precisely so I don't know more than the wikia.
Thought you might like the more detailed answer
