Drawing while casting

There's a lesser-known rule in Magic the Gathering that says:

401.5 If a spell or ability causes a card to be drawn while another spell is being cast, the drawn card is kept face down until that spell becomes cast (see rule 601.2h). While face down, it’s considered to have no characteristics. The same is true with relation to another ability being activated. If an effect allows or instructs a player to reveal the card as it’s being drawn, it’s revealed after the spell becomes cast or the ability becomes activated.

What this means is that if a card is drawn during the process of casting a spell (the process of casting a spell starts by the player revealing the card he or she is going to cast, and ends with its costs having been successfully paid, and involves calculating said costs in between), the drawn card is kept face down and considered to have no characteristics until the spell has been fully cast.

This rule doesn't come into play very often. To my knowledge there currently exists one single card in the entire game where this rule applies (and if I'm not mistaken, this rule was in fact added precisely because of this single card). The card in question is Chromatic Sphere.

Because the ability in Chromatic Sphere is a mana ability (it does not target and it adds mana to the player's mana pool) it can be activated when paying the cost of a spell, ie. during the casting process of said spell.

What the rule is thus saying is that if you activate Chromatic Sphere while casting a spell, the drawn card will be kept face down and considered to have no characteristics until the spell has been successfully cast (ie. it enters the stack.)

Now, an interesting question is: What happens if, for example, the drawn card is now discarded, still during the casting process of the spell? What if the drawn (and then discarded) card is, for example, Progenitus?

So, consider this situation: You start casting a spell. When it comes time to pay its mana cost, you activate the Chromatic Sphere, drawing a card (which in this case happens to be a Progenitus). You then activate the ability of a Bog Witch and discard the card you just drew. The question is: Will Progenitus be shuffled into the library or not?

The Progenitus card will be face down and without characteristics during this whole process (because the abilities of both the Chromatic Sphere and the Bog Witch are mana abilities, and activated here during the casting process of a spell). Since the discarded card has no characteristics, will its replacement effect happen?

(Note that the "reveal Progenitus" part doesn't exist any more than the replacement effect itself, if the card has no characteristics, so it can't be used as an argument for the replacement effect applying.)

The jury is still out on this one. It seems that this is an unclear situation, ie. a situation where the rules may not give an unambiguous answer. Some arguments can be made for the replacement effect happening, and other arguments for it not happening. The rules don't seem to give a definitive answer.

(In a tournament setting this would in practice be up to the judge to decide.)