Locations can always be challenged, and you dont need to wait for them to be exerted as they cant
2: Add for each Mountain you control
[20] On each player's turn, following a set phase order, they draw a card, tap their lands and other permanents as necessary to gain mana to cast spells, engage their creatures in a single attack round against their opponent who may use their own creatures to block the attack, and then complete other actions with any remaining mana
Hexproof creatures are still vulnerable to board wipes, but given that Snakeskin Veil can be played as an instant , you can use it to shut down a targeted spell first, and hope that your opponent doesn't have the mana or cards in hand to follow it up with anything else
A transforming double-faced card enters with its front face up by default, unless a spell or ability instructs you to put it onto the battlefield transformed or allows you to cast it transformed, in which case it enters with its back face up
This one is a little tougher to cheat, as you'll have to spend mana if you want to trigger its 'enters the battlefield' board wipe
