I'll answer your questions

I have "Pole Position", "Luminous Spark" (+500 to LIGHT), and "Gemini Elf" (1900 ATK / EARTH) face-up on the field. "Gemini Elf" is the monster on the field with the highest ATK, so it is unaffected by Spell Cards. If the opponent Summons "X-Head Cannon" (1800 ATK / LIGHT), "Luminous Spark" will raise the ATK of "X-Head Cannon" to 2300, making it the highest ATK monster on the field, so it's unaffected by Spell Cards, so it drops to 1800 ATK, then increases to 2300, then drops to 1800, etc. This would create an infinite loop. In this example, what card would be blocked from activating, according to the standard rules of the Yu-Gi-Oh! card game?