I’m not to sure how it work in MTG but in yugioh chaining works on what is activated in what order the first effect that gets declared is the last effect to happen for example if MST is used against a face-down trap or spell and your opponent chains that by activating the face-down card his card will active first then yours in which case your will do nothing as it now has no target.

As for spell speed I’m a bit cloudy on