Does a unit's summoning skill raise the level of the spirit? (Memetes, Phoenix etc.)
The level of a unit's summoning skill is the maximum cap of the rarity of the spirit. If a unit has a summon skill level of 2, the spirit cannot have a greater rarity than 2 stars. In order to summon a 4 star rarity Behemoth, a unit needs to have the summoning skill upgraded to level 4 and the Behemoth itself must have been evolved to 4 stars.
Are a spirit's abilities affected by the MAG of the caster?
The abilities are modified by the spirit's stats, not by the caster's. For example, Memetes' Guilty Ash damage is amplified by Memetes' MAG.
What is the relevance of summoned spirits' stats, such as HP, ATK, and cost?
These stats do not affect the summoned unit's abilities at the moment. However, these may affect summoned units' abilities in the future.
If two units cast the same ability, which one is applied?
If two (or more) units cast the same ability, they will resonate. When abilities resonate, they gain one additional level. This means that if 3 units casting the same level 2 Brave Burst, the second unit's Brave Burst will be level 3 and the third unit's Brave Burst will be level 4. This applies to all active offensive and defensive abilities, but not to leader skills.