Q1: Can I trash just 1 digivolution card for the conditions of this card's All Turns effect?

No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this All Turns effect require you to trash the specified number of digivolution cards.

Q2: If I use this card's On Play When Digivolving effect to place a card under a Tamer that already has cards under it, in what order do I place the cards?

If there are already any cards stacked under the Tamer, you place the new card at the bottom.

Q3: Can I use this card's On Play When Digivolving effect to place 1 of my opponent's Digimon under another of their Digimon, one that isn't affected by effects?

No, you can't.

Q4: If I use this card's On Play When Digivolving effect to place an opponent's Digimon under my opponent's other Digimon as its bottom digivolution card, will the Digimon to be placed in digivolution cards leave the battle area?

Yes, it will leave the battle area. The digivolution cards of the Digimon to be placed in digivolution cards will be trashed at the same time.

Q5: If I use this card's On Play When Digivolving effect to place an opponent's Digimon under one of their Tamers, does that Tamer gain the placed card's inherited effect?

No, it doesn't. A Digimon will gain an inherited effect, but a Tamer doesn't gain it.

Q6: If I use this card's On Play When Digivolving effect to place an opponent's BT12-038 GeoGreymon from the battle area as the bottom digivolution card of their BT17-087 Marcus Damon that's treated as a Digimon, does that BT17-087 Marcus Damon gain BT12-038 GeoGreymon's inherited effect?

Yes, it gains the inherited effect. If BT17-087 Marcus Damon suspends, BT12-038 GeoGreymon's inherited effect will trigger. However, as soon as it is no longer treated as a Digimon, it will lose inherited effects.

Q7: Does this card's All Turns effect trigger when either my Digimon or my opponent's Digimon gets linked?

No, it doesn't trigger.