Question 2 - can mage spend two gems on casting a spell, if he already can cast it with his base levels?
The answer to this question is actually "no". Or maybe "yes," depending on what you're asking exactly. There are two ways mages can spend gems on a spell. The first is to boost their effective path level for casting a spell. A mage can only spend 1 gem per spell to boost their path level, allowing them to cast spells at 1 level higher. The second way to spend gems is by paying the cost of a spell that costs gems (most battlefield summons and many battlewide enchantments or other big spells). Every mage has a per battle limit of how many gems they can spend, which is the level they have in the path of the gem. This limit will also be increased by anything that boosts the mage's paths for the duration of the battle (summon x power, crystal shield, communion, etc). There's no limit on bloodslaves.
For example, giving an E1 can spend an earth gem to cast Summon Earth Power(E2), boosting them to E2 (and also boosting their gem spending limit) for the rest of the battle, which then allows them to cast Strength of Giants(E3) by spending another gem, but then they're back to casting only E2 spells.
As another example, an A2 mage with 2 air gems can cast Arrow Fend. They spend 1 gem to temporarily boost their path to A3, allowing them to cast the spell, and the second gem is used to pay the cost of the spell.
Lastly, an F2 mage can spend 1 gem to cast Fireball as an F3, but can't spend 2 gems to cast Fireball as an F4, because gems can only boost path once per spell.