Can I use a different payment account for each store?

Like most payment methods, we only allow account configuration (API keys) at the website scope. On the settings page, change the 'Scope' dropdown at the top to a website, and you can enter different API keys there.

We don't allow these settings to be changed at the 'store view' scope, because Magento allows carts to be shared among store views in a website. This means a customer could create a cart on store A (with certain payment configuration), then switch to store B and pay for those items, and the money would go to that different account.

We don't have plans to change that behavior, but you could modify the settings to allow them to be changed at the store view, or you could add records directly to database table core_config_data to enable/disable in the respective store views.