If I have a player who finds some scrolls, and then casts read magic to decipher them, it is clear that he understands them. Could he in turn hand them to the rogue, who would otherwise need to use UMD, and explain it to him, or would the rogue have to decipher it himself with UMD or Spellcraft?
Being told what a scroll is supposed to do, should suffice. Adventuring is a team sport after all.
Not specified. I'd probably apply a bonus to the rogue's Spellcraft check (+2, similar to Aid Another). The rogue would still need Use Magic Device to emulate having the scroll on his spell list and/or to emulate having the requisite ability score.
Might I recommend any number of inexpensive wondrous items that provide read magic? Alternately, the minor magic rogue talent was basically made for this.