So from my understanding you need to declare the spell you are casting, but will forgo it for a summoning point. Note (also my understanding) you need a unique spell each time you wish to forgo said spell, you cant attempt arcane bolt 3 times and say they will become said summoning points. If you cast a spell like normal you get no points from it. Also note that attempting to cast a spell, then making it a point instead does not make it eligible for unbinding from the opponent as no spell was actually cast, same goes for rolling dice for said attempt.
i've never done such a thing. Usually i simply say "i forego 2 spells and i cast quicksilver swords". Given that the Slann knows 3 spells (at the very minimum), it's fairly trivial to declare what spell you are going to "not cast"
If you do not make a die roll as part of the attempt to cast the spell it does not count as an attempt, and thus the spell can be attempted "again" by another caster. you do not have to declare the spell you aren't casting, and you do not lose the ability to cast the spell you didn't cast later in the phase.