Rector Tendrills questions

[bedafile]

My brother and I have been working on mastering a rector tendrills deck for an upcoming toournament and I have a few questions about playing it.

Can I play Mox Diamond, choose to not discard a land from my hand and still tap it once for mana on the way to the graveyard?

My brother also pulled this move, I was wondering if it works. Academy Rector is cast with Cabal Therapy in the graveyard. I cast Swords to Plowshares on his turn targeting Rector, hoping to remove it before he can use Cabal Therapy. He claims that instants can only be put onto the stack or played on my turn so at this point its not a valid target yet. He flashbacks Cabal Therapy sac-ing Rector. He claims I can't STP his Rector at this point either because that's part of Therapy's casting cost. This seems a little suspect, is this claim valid that there was nothing I could to remove Rector with STP?
Thanks for the help.

[Toad]

You discard the land as part of Mox Diamond's casting cost. So if you don't discard a land, you simply can't play it. Mox Diamond's has been errated.

As an additional cost to play ~this~, discard a land card from your hand. ; : Add one mana of any color to your mana pool.

Second question. You have no opportunity to STP the Rector. Your opponent casts a Rector, then receives priority. Then he plays Cabal Therapy saccing the Rector. Then you receive priority, but the Rector is already in the graveyard.

[rvs]

First: Discarding a land for Mox Diamond is a COST. This means you cannot tap it for mana since it won't come into play yet. This was an update from 6th ed. rules, IIRC.

Second: In order to play STP, you must have priority. If you do not have priority, you cannot play it. If your brother manages to put Therapy on the stack before you have priority to STP it, you cannot anymore, as saccing the creature is a casting cost of the flashback therapy. However, don't forget Therapy is a sorcery, so you can just Swords to Rector during his upkeep or something. Just don't let the game go into the first mainphase of your brothers turn, since then HE will have priority.

Hope this clears things up a little.

[DEA]

He claims that instants can only be put onto the stack or played on my turn so at this point its not a valid target yet.

i believe this is the part that is VERY FISHY
instants may be played anytime you have priority, regardless of whether the stack is empty or not

however, like toad and morefling have said, once rector resolves, he has priority, and if he doesn't pass, and flashes therapy, your stp is as good as a wrath of god

next, your opponent HAS to search out the enchantment before naming a card
it's normally not nice to cheese the opponent this way, but if you're in a tournament, go ahead