1. You need to tooltip scan and account for different locales. No other known way.
2. Not possible. Also, it is inaccurate that "base mana = maxmana - amount added by intellect as per tooltip" because mana is also affected by talents and racials that increases your max mana and/or intellect, and these become white colored text which aren't accounted for. That is to say, a Gnome's casting cost of a spell isn't any higher than a Human just because his base mana is 5% higher.
3. Not possible either. You need a lookup table using data from WoWHead or otherwise. The enchantID has nothing to do with the spellID, it is just an arbitrary number for the internal tooltip code to show the correct text for the green-colored enchant line. WoWEquip maintains such a list (until patch 3.0 broke it)
4. Tooltip scanning again. Try looking in WoWEquip again, and/or LibItemBonus-2.0, both of which do such tooltip scraping. Note the addon doesn't actually work in patch 3.0.