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You have total cover inside the creature. That means the point of origin is on the inside, because when you have total cover, so does the outside, and area effects are blocked if there is no clear to the target:

The space within the creature is not well defined. Since the obstacle is also a creature that can potentially be affected by the area spell I’d check with my DM as to how they want to handle this unusual situation, and if they want to apply the effects to the creature too, or only the inside of the creature.

You have total cover inside the creature so it provides an obstruction to any point outside creature, and any point outside likewise has total cover. That means if you pick a point of origin on the outside, it instead will end up on the inside, and the area would expand from the inside. Area effects however cannot expand either, if there is no clear path:

Since the obstacle here is also a creature that can potentially be affected by the area spell I’d check with my DM as to how they want to handle this unusual situation, and if they want to apply the effects to the creature too, or only the inside of the creature.

Since you have full cover inside the creature, that means the point of origin is on the inside.

The space within the creature is not well defined. Area effects would likely be limited to the space within, because when you have total cover, so does the outside, and area effects are blocked if there is no clear to the target

Since the obstacle is also a creature that can potentially be affected by the area spell I’d check with my DM as to how they want to handle this unusual situation.

You have total cover inside the creature. That means the point of origin is on the inside, because when you have total cover, so does the outside, and area effects are blocked if there is no clear to the target:

The space within the creature is not well defined. Since the obstacle is also a creature that can potentially be affected by the area spell I’d check with my DM as to how they want to handle this unusual situation, and if they want to apply the effects to the creature too, or only the inside of the creature.

Flaming sphere would not work, as it says:

So, check each spell for what the requirements for sight and target are.

You need to check each spell for what the requirements for sight and target are.

Flaming sphere would not work, as it says:

Tidal Wave can be cast as it has no such limitations, but it is more difficult to determine what then happens. Its area of effect is

You conjure up a wave of water that crashes down on an area within range. The area can be up to 30 feet long, up to 10 feet wide, and up to 10 feet tall.

While this is not one of the standard areas of effect (cone, sphere, line, cube or cylinder), it still is an are of effect. The targeting rules say (p. 204, PHB)

If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

Since you have full cover inside the creature, that means the point of origin is on the inside.