In any case, that last question is covered in the rulings, sometimes it helps to read the rulings for a card first....
You cannot Tribute a monster for "Magical Dimension" that is "unaffected by Spell Cards" since Tributing is part of the effect of "Magical Dimension" (not a cost). If the only monster on your side of the field is "Gemini Elf" while "Non-Spellcasting Area" is active, then you could not activate "Magical Dimension".