Areo Hotah

In a melee game, if Areo enters play during a challenge between the other 2 players, can his ambush reaction still be used and one of the participating characters removed from the challenge even though the Martell player is neither the attacker nor the defender in the challenge?

Yes —
The Red Viper

If you win a challenge in which you control two attacking bastard character, can you trigger reaction twice?

no. reactions and interrupts can only be triggered once per their triggering condition, which in this case is winning with attacking Bastard character —
Ser Meryn Trant

Does playing an event count as discarding a card from hand for the purposes of this effect? If so, when does the card count as being discarded - eg if meryn is put to the sword.

No, playeing an event is not discarding a card. —
However, some events have cost of discarding cards, and then those cards you could trigger Meryn on. —
Ser Barristan Selmy

Rules question.

If I choose to save a Lord with Ser Barristan, but Ser Barristan is cancelled, can I then use a duplicate to save said Lord instead?

Very late reply but the answer is YES. If cancelled Selmy will Stand and the character may be saved by another means. —
Melisandre's Favor

Does the character return to the original owner, following the forced interrupt? I would think so, but i'm not certain

Yes, it does return. Also see the review on "Ward" for a detailed look at taking (and losing) control of characters. —