Can you attach cards if their Sphere of Influence is different, so long as you paid for the card using the appropriate resources?

Example: If I use resources from Eowyn's pool to pay for Unexpected Courage, could I attach it to Aragon?

Yes, the sphere of an attachment has no bearing on who it may be attached to.

While reading the Preview 'Sacrifices in The Hills of Emyn Muil' I came to the following:

'Players can play attachments on heroes other than their own'

Is this correct? We always played attachments on the player playing the attachments.

It is indeed, and has been a part of the game since the core game. For example, look at the attachment 'The Favor of the Lady'. It simply says 'Attach to a hero.' not 'Attach to a hero you control.', therefore you can play it on any active hero in the game regardless of which player controls that hero. This goes for any card in the game as well, not just attachments. Unless a card specifically uses the phrase 'you control' in the text, then it can be used to affect any players cards.