[Endoperez]

Yeah, a similar formula is used for everything.

Magical weapons add strength when there's no "str not added" tag.
Magical bows have better chance of getting a hit through shield etc, as page 77 mentions.
Spells deal the damage listed in their description, with just Farstrike and perhaps Fists of Iron adding str. The little + after damage means that having more power in the path used in the spell's casting increases damage. Exactly how much varies depending on the spell, but it's about 1-3 points/extra level or so.