You cannot perform a Flip Summon during your opponent's turn. A Flip Summon is a way of manually changing the monster's Battle Position. Since it is "manual", it can only be done during your own Main Phase.
You cannot use Ultimate Offering to Flip Summon a monster during your opponent's turn. Ultimate Offering allows you to Normal Summon or Set - while a Flip Summon counts as a Normal Summon, it is not exactly a Normal Summon.
Any force-flip does not count as a Flip Summon. If a face-down monster is attacked during your anyone's Battle Phase, it is considered "flipped face-up" and is not Flip Summoned (or summoned in any way). If your opponent uses Stop Defense, this is not considered a Flip Summon (or any kind of Summon). If your opponent plays Swords of Revealing Light, your monster is not considered Flip Summoned. And so on.
You cannot use Ultimate Offering to Flip Summon a monster during your opponent's turn. Ultimate Offering allows you to Normal Summon or Set - while a Flip Summon counts as a Normal Summon, it is not exactly a Normal Summon.
Any force-flip does not count as a Flip Summon. If a face-down monster is attacked during your anyone's Battle Phase, it is considered "flipped face-up" and is not Flip Summoned (or summoned in any way). If your opponent uses Stop Defense, this is not considered a Flip Summon (or any kind of Summon). If your opponent plays Swords of Revealing Light, your monster is not considered Flip Summoned. And so on.