The only further clarification I have is I think both responses have eluded to the fact that Insidious Dreams would be cast first before the madness triggers, but the Gatherer Rulings for madness cards state:
"If you discard a card with madness to pay the cost of a spell or activated ability, that card’s madness trigger (and the spell that card becomes, if you choose to cast it) will resolve before the spell or ability the discard paid for."
(I get that the whole point is moot and that the entire action would ultimately be illegal, but it's a procedural clarification I want to make)
My understanding of the spell casting structure is that you can put Insidious Dreams on the stack and choose to discard the 3 Alms of the Vein before tapping the Swamps, which will activate their Madness effect immediately. I can then use the 3 Swamps I have available, opponent loses 9 life and I gain 9 life. Here's the tricky part: I return to Insidious Dreams, still on the stack, to finish casting the spell and I now don't have enough mana (although I did when this started). Insidious Dreams would fail to resolve but I'd still get to cast the 3 Alms of the Vein.
"If you discard a card with madness to pay the cost of a spell or activated ability, that card’s madness trigger (and the spell that card becomes, if you choose to cast it) will resolve before the spell or ability the discard paid for."
(I get that the whole point is moot and that the entire action would ultimately be illegal, but it's a procedural clarification I want to make)
Let's say you have one Insidious Dreams and three Alms of the Vein in your hand with only 4 Swamps available.
My understanding of the spell casting structure is that you can put Insidious Dreams on the stack and choose to discard the 3 Alms of the Vein before tapping the Swamps, which will activate their Madness effect immediately. I can then use the 3 Swamps I have available, opponent loses 9 life and I gain 9 life. Here's the tricky part: I return to Insidious Dreams, still on the stack, to finish casting the spell and I now don't have enough mana (although I did when this started). Insidious Dreams would fail to resolve but I'd still get to cast the 3 Alms of the Vein.
Legal?