Q1: Only the top card of my opponent's security card is face up. If this card's When Digivolving effect activates, what security card is flipped to face up?

The 2nd card from the top of their security stack is flipped to face up.

Q2: What happens to cards placed face up in the security stack by effects?

They become face-up security cards that remain revealed. Other than rules that specify face-up security cards, the rules apply in the same manner as standard security cards.

Q3: What happens upon a security check for a security card that is placed face-up?

The check is performed with the card left revealed. Other than rules for cards left revealed, the rules apply in the same manner as standard security checks.

Q4: Does a card's Security effect trigger upon a security check with that card placed face-up?

Yes, it triggers.

Q5: What happens if I shuffle a security stack that includes security cards placed face-up?

Any face-up cards are placed face down, then you shuffle the cards. After shuffling, all cards are left face-down.