When I’m adding opposing characters’ {S} values together to get the total {S}, do I take into account any effects that modify {S}?
Yes. You’ll use the current {S} values of the characters. For example, if your opponent has a 2 {S} character that gets +1 {S} from an effect, that character’s current {S} value would be 3. It’s important to note that if a character’s current {S} would be less than 0, it’s considered to be 0 for this calculation.
Official set release notes — Wilds Unknown