If Meandering Towershell enters the battlefield attacking, it wasn’t declared as an attacking creature that turn. Abilities that trigger when a creature attacks, including its own triggered ability, won’t trigger.

On the turn Meandering Towershell attacks and is exiled, raid abilities will see it as a creature that attacked. Conversely, on the turn Meandering Towershell enters the battlefield attacking, raid abilities will not.

As Meandering Towershell returns to the battlefield because of the delayed triggered ability, you choose which opponent or opposing planeswalker it’s attacking. It doesn’t have to attack the same opponent or opposing planeswalker that it was when it was exiled.

If you attack with a Meandering Towershell that you don’t own, you’ll control it when it returns to the battlefield.