The rules for item creation seem to indicate that you can always share prerequisites. The wording of the feats always is about "you" meeting the prerequisite or knowing the spell, but the rules for item creation say this:
I suppose it could be interpreted differently, but the feat statement would seem to be modified by the caveat in the item creation rules, above. This is supported by the identical language from the various creation feats. For scrolls, potions, wands, and staffs, that verbiage is as follows:
The only difference between that and the other items (rods, wondrous items, weapons, armor) is the addition of the phrase, "If spells are involved in the prerequisites for making the [item]." (Oddly, rings have different verbiage.) This distinction is likely because there are some items from that set that do not have spells as a prerequisite, whereas potions, scrolls, and the like all have spells as prerequisites. Those prerequisites still seem to fall under the umbrella of the parenthetical "... access through another magic item or spellcaster is allowed."
Though the quoted text is from the online SRD, I found similar language in my perusal of my (3.5) DMG.
My recollection is a distinction between spell completion and other items, but perhaps the clarification is in Pathfinder rather than in 3.5. There is another difference between spell completion and other items, however. The material components and xp costs must be incurred for spell completion items, while for other items, the creator "need not provide any material components or focuses the spells require, nor are any XP costs inherent in a prerequisite spell incurred in the creation of the item".
The scroll rules are more specific that "The act of writing triggers the scroll", so what does the person with the Scribe Scroll feat do, if they do not in fact write the scroll?
Here we go -
http://www.d20srd.org/srd/feats.htm
Any potion, scroll, or wand that stores a spell with a costly material component or an XP cost also carries a commensurate cost. For potions and scrolls, the creator must expend the material component or pay the XP cost when creating the item.
For a wand, the creator must expend fifty copies of the material component or pay fifty times the XP cost.
This does not apply to other items, and seems to imply that the creator must be the caster - no one else can pay the xp cost of the spell.
Further, read the text of the feat. You may Brew a potion "of any 3rd level of lower spell that you know", scribe a scroll "of any spell that you know", and create a wand "of any 4th-level or lower spell that you know". The other crafting feats allow you to create a relevant item "whose prerequisites you meet", and do not refer to you specifically knowing spells. Thus, the ability to provide a spell you don't know can meet the prerequisites of the feats other than potions, scrolls and wands. The last three mention specifically knowing the spell, and not meeting prerequisites.
I agree there is some lack of clarity, leaving this in the feat but not specifying it in the crafting rules. However, we are back to interpretations that, if they do not clearly contradict the rules, are clearly the most favourable interpretation possible to the spellcaster.
How is casting time a "heavy handed restriction"? The player knows what it is, and plans and declares actions accordingly.
Sorry, I should have put the phrase in quotes, as pretty much every read of the rules that does not favour the spellcaster to the maximum extent possible (or even override the clear words in favour of the caster) has been indicated by posters other than yourself to be "heavy handed restrictions" on the spellcasters. Still, while I would suggest making all spells have a minimum 10 minute casting time would clearly move the balance in combat to the fighter, it would presumably be viewed as a very heavy-handed manner of implementing balance.
To the specific issues, I would say that Weather Sense and Weather Control should be balanced exactly against one another if each requires an equal dedication of character resources. If each requires, say, one skill point per level and nothing else to max out, they should be of equal utility.
I would go further: it requires abandoning the module. The only support that White Plume Mountain provides to the "ally with Keraptis" scenario is providing a name. The whole rationale of the module is for the players to take their PCs in and beat the dungeon.
I suggest there is no question that no module ever written can cover every possible reaction or approach by the players and, as such, full player freedom equates to being willing to modify or abandon parts of, or even the entirety of, the module. There is an aspect of railroading in any module. It is simply a question of how much. The module would be just as abandoned if the players decided they weren't motivated by the promise of the loot in the dungeon, and went looking for other work. There is a general presumption of widely shared motivations of PC's in pretty much every module, although more recent modules tend to suggest a broader array of possible hooks and/or provide guidance to players to create characters who will be motivated to participate in the module.
As a longstanding example, how many groups prohibit evil characters, at least in part because the scenarios will involve a battle against evil? How would the Giants modules play out if the players desired to join (or lead) the giants, or offered their services to Llolth? I think most modules provide one or more expected adversaries to the PC's and don't typically envision the PC's deciding to ally with those expected adversaries (much less successfully forge such an alliance). Doubtless there are occasional exceptions, much like there is the occasional module designed for solo play, which minimizes combat and/or is designed for Evil characters, but the norm is to provide adversaries to the PC's in some form or another.
