Why doesn’t the Wailmer hatch with Aqua Ring

The Power of an attack directly correlates to how much damage you deal, because its presence in the damage formula is entirely multiplicative. So, an attack with 120 power will do twice as much damage as one that has 60 power. The same goes with your corresponding attack stat: a Pokemon with 100 Attack will do twice as much damage as one with 50 Attack when using the same attack on the same target.

STAB has operated consistently since Gen 1 so this applies to all of the games. It increases the Power of an attack by 50% if it matches one of your types. 60 becomes 90, for instance.

Technician increases the Base Power of an attack by 50% if its Power is 60 or less. So the most you get is boosting an attack at 60 power to 90. This occurs before STAB is calculated, so your Power would be 90 for STAB, boosting it to 135.

I dusted off my old copies of Pokémon Diamond, Platinum, and SoulSilver to test this. When you enter a trade in the Union Room, every Rotom in your party turns into a normal-forme Rotom, even if it isn't being traded to another game. The appliances get returned to the room in each respective game. I suppose Gamefreak anticipated this issue and implemented this to make sure people wouldn't run into any same Rotom-forme problems.

It's not possible to have two Rotom of the same forme. When you choose an appliance to turn Rotom into a specific forme, the appliance then disappears (until choosing a different forme), so there wouldn't be a way to have two of the same forme in your game without cheating.