Q1: I attack with this card, and gain 3 memory from the card's attack effect. If this Digimon is deleted as a result of the attack, do I still lose 3 memory at end of turn?

The effect has already activated, so you lose 3 memory at end of turn.

Q2: I attack with this card, and gain 3 memory from the card's attack effect. If I then announce that I'm passing, how much memory do I lose?

Announcing the pass moves the memory counter to 3 on your opponent's side. Since it's the end of the turn, you then lose an additional 3 memory, moving the memory counter to 6 on your opponent's side.