1.) Do attachments kneel with a character when they attack or defend in a challenge? For example can I attack with a character equipped with lightbringer and then later that turn use lightbringer to save them from being killed after a failed military challenge defense

2.) When a card (like Dany's handmaiden) says choose and discard an attachment from play, does that mean ANY attachment or just an attachment you control?

1. no, the atachments stay standing, unless something specifically says to do so (i.e. your example of Lightbringer)

2. Unless it specifies, it means any. Unless it's something like "Discard a [Targaryen] atachment to save a character you control." Then it has to be one you control, because you are paying a cost, and you can't use an opponent's cards to pay a cost for your effect.

Expanding on what Stormwolf said:

Storm is referring to the difference between paying a cost and a card's effect.

"Discard an attachment to..." - Note that the majority of the time anything before "to" is a cost. Costs must be paid with cards you control regardless of whether or not the text of the card specifically states so.

"...to discard an attachment from play." - In this example, everything after "to" is the effect. This is not a cost and so any and all restrictions must be explicitly stated as qualifiers or there are no restrictions. In this example, the only restriction is that you must discard an attachment (and not another type of card).