Hello.

White Sword Tower: "Kingsguard characters get +2 STR and do not kneel to defend during power challenges".

Is the "+2 STR" always valid, or just during power challenges? How can I tell such wording, in general?

It would be much clearer if written:

"Kingsguard characters get +2 STR. Kingsguard characters do not kneel to defend during power challenges".

or

"During power challenges Kingsguard characters get +2 STR and do not kneel to defend"

depending on the case.

Thank you.

The +2 is always valid. Any modifier that comes before applies to everything after. For the +2 to only come during POW challenges, the effect would have to be worded the way your second hypothetical is worded.