BTW, note the fire in background: Elliot CAN use illusion to make such effect. Liz presumably can't, unless THAT is the spell she got from Pandora.
Yeah sure. And I would expect he already sparred with Ellen as well.
No, really, her being girl would make little difference. The important think is that as martial artist, he is trained to not use violence on untrained people. Presumably.
He didn't had one in the age where she would be unbearable without using physical violence on her.
In their age, people generally know when to stop (usually because someone already used physical violence on them earlier ; for lot of people in school that someone was Elliot). Even if they don't know each other almost as good as themselves.
I don't think the hints Liz got would point to this possibility. Also, I would expect the "other possibility" will include the party somehow. Him being gay would have no effect on the party.
And more relevant for his relationship with Ashley, he's totally lesbian.