Q1: The situation is that only the top card of the opponent's security is face-up. If you activate the When Digivolving effect of this card, which card of the security should you turn face up?

Turn the second card from the top of the security face up.

Q2: What happens to a card placed face up in security by an effect?

It becomes ostensible security and remains public. The rules are the same as for normal security, except that it is ostensible.

Q3: What happens when checking a security placed face down?

Checked as published. The rules are the same as for normal checks, except as published.

Q4: If a security placed face up is checked, does the card's Security effect trigger?

Yes, triggered.

Q5: What happens when shuffling a security that contains a security placed face up?

The security that was face up is turned back face down and then shuffled. After shuffling, the cards are not turned face up.

Q6: If the Security effect, the "when security is checked" effect, and the "when security is reduced" effect are triggered simultaneously, in which order do they activate?

Security] effects are activated first. Security] effects, if checked, will be activated immediately without pending activation. Other triggered effects are activated from the turn player's side.

Q7: I attacked with this card and checked the security face-up. At this time, if I activate this card's Your Turn effect and place one of the stacked cards from the top under the security, does it trigger the End of Attack effect of the Digimon that was under this card?

Yes, triggered.