But what if you try to cast it anyway from exile? By the time you check for legality, it's no longer in exile but on the stack...
A friend is trying to convince me this is possible but I'm uncertain. I think 101.2, "When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence." is relevant - since you'er not allowed to cast the Griffin...
As a note, you also still get the Wurmcoil tokens. You can even choose the relative ordering of the Scrap Trawler triggers and the Wurmcoil trigger though I'm not really sure it matters in this scenario.
That is a standard trigger condition. It triggers whenever 'a creature of the chosen type enters the battlefield' or 'a creature of the chosen type attacks'. If multiple such creatures enter the field at once, or attack at once, Kindred Discovery triggers that many times. So you draw 2 cards in each of your questions, assuming you chose Soldier as the chosen type.
Also, just to clear things up, the tokens would have to wait till next turn to attack for they don't have haste and a creature that entered can't attack its first turn.
You'd deck yourself. If Mairsil cages the Phantasm, it's ability now reads "1U: Mairsil the Pretender's owner shuffles it into his or her library. If that player does, he or she reveals cards from the top of that library until a card named Mirror-Mad Phantasm is revealed. The player puts that card onto the battlefield and all other cards revealed this way into his or her graveyard."
You won't find one, of course, for your sole copy is in exile. The rules about replacing names in abilities only refer to self-referential ones like the first instance of the Phantasm in that ability - if you see 'cards named' or something like that, that's not changed. You'll end up with your whole library in your grave and probably end up facepalming.
Even if you move Mairsil to the command zone instead of actually putting it into your library, you'll deck yourself. (You might want to not do this.)