[Saulot]

Well... according to my manual, it requires an astral 5 mage and it costs 500 fatigue. Which translates to 5 astral gems required.

To answer your first question: A mage can use as many gems in a path as they have that path in question. A mage can use gems to boost his level temporarily to cast a certain spell (for that spell only). IE. an astral 2 mage, casting the astral 3 spell Antimagic (using 1 gem to become astral 3, and one gem to cast the spell).

Since Solar Brilliance requires 5 gems, no mage under astral 5 could cast it.

This could be achieved by magic item boosters (base astral 4 + astral +1 item) or through communion, or through spells which increase paths. Obviously some things may be more difficult to accomplish than others.

As for the second question: The most obvious solution seems to be to cast Antimagic beforehand, which should make your units less likely to be affected by the blindness, but it still won't be perfect.

As for the third: the spell teleport works perfectly fine. There are a couple 'relocate/teleport' spells which sometimes have problems, and possibly a few magic items which also gave 'relocation' which also may not work until a particular spell is researched. I believe that a few of these bugs may have been fixed in the upcoming patch.