Can I pay a "when this attacks, you may pay energy" cost multiple times?
What you have there are examples of triggered abilities. They say "Whenever [this card] attacks, ..." and this means that the moment you declare them as attackers, the ability triggers once and only once, and it's put on the stack.

When the ability resolves you follow the text of the ability: you either pay the mentioned energy cost or you don't. You don't have the option of paying it several times in a single trigger because the ability doesn't say you do. And it doesn't trigger multiple times because "Whenever [this card] attacks, ..." is specifically a simplification of "When you declare [this creature] as an attacker, ...".

You would have been able to pay an arbitrary amount of energy if either of the following had been the case:


If the ability were an activated ability. Activated abilities look like "Cost: Effect". So if the swooper had had "[2 energy symbols]: Create a 1/1 artifact servo token", you could've created an army at any time, provided you had enough energy.
If the triggered ability had explicitly let you pay as much energy as you wanted. If the leopard had had "Whenever Aetherborn Leopard attacks, pay any amount of [energy symbol]. Aetherborn Leopard gets +2/+0 for each [energy symbol] spent this way" you could've pumped it to be as large as you wanted, provided you had enough energy.