Is it legal to take an action that requires me to search my library, pull out a Panglacial Wurm, say I'm going to start casting it, activate Chromatic Sphere's Mana ability to draw the top card of my library (which is now known to me because of the search), and then say "oops, I can't fully pay for this card, putting the Wurm back but still getting to somewhat cheese the draw, since that can't be reversed?
if you find you're unable to cast a spell after starting to do so, that action is reversed as further detailed in C.R. 722.1 (see also C.R. 601.2), but
actions that moved cards from your library to your hand, including drawing a card, can't be reversed this way (C.R. 722.1, 121.1, 400.1).
Issues involving Panglacial Wurm were discussed in this thread and this thread.
EDIT: Edited after comment 3 was posted.
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